A comprehensive review of Trinitor components: A sustainable waste heat recovery polygenerative system for diesel vehicles

Abstract

Internal combustion engine inefficiencies and waste heat emissions raise environmental concerns, as they waste fuel energy in the form of heat, increasing fuel consumption and greenhouse gas emissions. Additionally, waste heat contributes to the urban heat island effect. Waste heat recovery is a vital solution, capturing and repurposing heat to reduce fuel use, emissions, and costs while promoting sustainability, innovation, and economic growth. Polygenerative waste heat recovery maximizes energy efficiency by generating multiple forms of energy from a single source, enhancing overall sustainability. The proposed Trinitor model is a polygenerative system encompassing power generation, product drying, space cooling/heating, and oxygen production. Power generation utilizes exhaust heat stored in a phase change material (PCM) to generate electricity through a Hot Air Turbine. The PCM also stores heat from the PVT thermal collector and supports produce drying. In the space cooling/heating process, the temperature contrast resulting from the hot air generated by the turbine and the cooled air from the Cooling chamber is harnessed by the Seebeck principle within the TEG, converting heat energy into electricity, and it is possible to create temperature variations using the Peltier Effect by supplying electricity. Oxygen production involves dehumidifying air, separating oxygen from hydrogen using an electrolyzer and storing oxygen for civilian use. A component review identifies SiC wall flow-diesel particulate filters (DPF), a paraffin-based Latent Heat Storage System, and electric-assisted turbo compounding as cost-effective for energy production. Produce drying relies on hot air or infrared drying, a revolving wicks humidifier, and a cooling coil dehumidifier. Space cooling/heating needs a water-type PV/T collector, MPPT charge controller, lithium-ion batteries, and ceramic TEGs. A PEM electrolyzer with appropriate components (bipolar plates, electrodes, catalyst, membrane, and gasket) enhances oxygen production efficiency. Based on existing literature, the trinitor has the potential to attain an overall efficiency ranging from 40.12–54.81%. Thus, a combination of low-efficiency processes results in a highly efficient waste heat recovery Trinitor system, with further improvements possible through identified components’ integration.

Introduction

There has been a tremendous improvement in every part of society in the modern world. The production of electrical and thermal energy is driving this improvement in living conditions and human potential. The tremendous growth in energy demand over the past few decades has been met by fossil fuels. In 2018, renewables, nuclear energy, hydropower, natural gas, coal, and, oil accounted for 4%, 4.4%, 6.8%, 23.9%, 27.2% and 33.6% of global primary energy consumption, respectively [1]. However, in 2018, fossil fuels provided 84.7% of the world’s primary energy demand. It is hoped that CO₂ emissions would be reduced by 37 gigatons per year by 2050 [2]. Nations and organizations are confronted with an energy tri-lemma, which combines concerns about energy affordability and economics with a goal for environmental preservation and guaranteed energy security. Addressing these three concerns at the same time has proven to be a challenging task for energy planners [3]. There is a distinct flow from the Middle East, West Africa, and Russia to the USA, Europe, and China in the global crude oil trade, which in 2019 reached a volume of 2239 million tons. Concerns about high crude oil dependence have been raised by the large gap between oil suppliers and oil buyers of the world. This has led oil-importing countries to improve efficiency, diversify oil supply sources, and promote renewable energies in order to lessen the impact of supply and price shocks, even if it is in an area that can’t be changed, like military and defense [2]. Thus, a dire and urgent need for reform is required in automobile applications and every country in the world has already started working toward that direction.

The issue of waste heat in internal combustion engines and its environmental effects is a major concern in the automotive industry. Waste heat is the unneeded heat produced during engine combustion that is released into the environment through the exhaust and cooling systems. The following are some of the issues and environmental consequences linked with waste heat. Internal combustion engines are inefficient because they waste a significant amount of fuel energy as heat. This inefficiency increases fuel consumption and greenhouse gas emissions. For example, gasoline-powered vehicles convert just 20–30% of fuel energy into work, squandering the remainder as heat [4]. Inefficient engines use more fuel to produce the same amount of power, increasing prices and producing more carbon dioxide (CO₂) and pollutants, contributing to global warming and air pollution. The release of waste heat has an impact on the local and regional ecology. Hot exhaust emissions can heat up cities, adding to the heat island effect. Waste heat release in cities can amplify the urban heat island effect, raising city temperatures and influencing air quality, energy consumption, and liveability. Engines require cooling systems, which consume energy to handle heat and so reduce overall efficiency. Cooling and exhaust systems are used to remove heat and generate noise, both of which have an impact on vehicle occupants and the environment [5].

Waste heat recovery stands as a crucial strategy that tackles both air pollution and energy efficiency issues [300]. It assumes a pivotal role in capturing and reusing heat that would otherwise be released into the environment. This method substantially reduces the necessity for additional fossil fuel burning, resulting in decreased greenhouse gas emissions and improved air quality. Waste heat recovery systems also enhance energy efficiency in various sectors like industry, power generation, and transportation by harnessing thermal energy that would otherwise go to waste. This leads to a reduction in the consumption of primary energy resources, which lowers costs and lessens environmental impact. Moreover, it aids in adhering to strict environmental regulations, offers cost savings to businesses and consumers, and contributes to resource preservation. Additionally, it fosters innovation in energy efficiency and clean energy solutions, paving the way for new industries, job opportunities, and economic growth. In summary, waste heat recovery is a multifaceted approach crucial for achieving a sustainable and environmentally friendly future [6]. Figure 1 shows the Environmental impact of waste heat and recovery methods.

Fig. 1

Environmental impact of waste heat and recovery methods

Diesel engines have been recognized to play a vital function in automobile sector. In recent years, extensive research has focused on enhancing diesel engine performance and waste heat recovery by integrating various components. Here is a condensed overview of the traditional approaches such as thermoelectric generators, bottoming cycles, turbo-compounding and heat exchangers used in engine waste heat recovery. These solutions are advantageous not only for huge vehicles, but also for the transportation sector as a whole. A thermoelectric generator (TEG) is a solid-state device that converts heat transfer into electrical energy by using conduction or semiconductor principles. TEG has a number of advantages, including its environmental friendliness, low noise level, lack of moving parts, and low maintenance costs. However, it only becomes economically viable when running at high temperatures with a limited power output. The authors conducted a mathematical model simulation to assess the performance of a TEG (TEP1-1264–3.4) in waste heat recovery (WHR). They used 126 modules operating within a temperature range of 248–396 °C and achieved a power output ranging from 185–605 W with an efficiency between 3.3–4.1% [7]. In another simulation, Y. D. Deng et. al applied 240 modules of Bi₂Te₃ TEG to 11.2 L diesel engine with an operating temperature of 115 °C, resulting in a power output of 995 W at 4.1% efficiency [8]. Furthermore, the authors conducted tests on TEGs (TG12-4) using a testing bench equipped with 100 modules at 125 °C, which yielded a power output of 130 W with an efficiency of 4.1% [9].

Waste heat recovery can also be achieved using secondary thermodynamic cycles called “bottoming cycles” to capture and use low-grade waste heat from primary energy operations like internal combustion engines and industrial processes. These cycles run at lower temperatures and pressures than the primary process to extract work or heat from wasted heat. Organic Rankine Cycles (ORCs), stirling cycles, and other heat-to-power systems aim to boost energy efficiency. Teng et al. conducted a simulation to recover waste heat from the exhaust gas of a Cummins ISX engine. They employed a T-type expander and a serpentine heat exchanger, with R245fa and ethanol as the working fluids. This approach yielded an efficiency ranging from 15.8–25.5% [10]. Cipollone et al. successfully recuperated waste heat from the exhaust gas of an IVECO NEF67 engine. They achieved this using a finned coil type evaporator and a T-type expander, and the working fluid of choice was R245fa. As a result, they were able to generate a power output in the range of 1–3 kW, exhibiting an efficiency between 9–0% [11]. Yutuc investigated the viability of integrating a Stirling engine into the exhaust system of a tanker’s engine. The primary goal was to reduce fuel usage while also providing power for the dashboard. Although the Stirling engine model was not specified, they used a conventional cycle efficiency range of 20–50%. The Malmo formula was used to calculate the power generated by the Stirling engine [12]. Douadi et. al developed a mirror cycle that combines a cooled-inverse Brayton cycle with a multi-stage intermediate cooling technique. When compared to a standard gas turbine cycle, their research found that the mirror cycle can achieve significantly higher thermal efficiency [13].

Turbo compounding uses an additional turbine connected to the engine’s exhaust system to recover energy from exhaust gases. This recovered energy is transformed into mechanical power to drive auxiliary equipment like electric generators or propellers in aircraft and ships. Heat exchangers are crucial to engine waste heat recovery. They are essential for transferring thermal energy from hot engine exhaust gases to other fluids or systems, enhancing engine efficiency and lowering energy waste. In their WHR experiment, Khordehgah et. al concluded that plate heat exchangers and heat pipe systems effectively transfer heat between sources with varying temperature ranges [14]. Table 1 lists some unique WHR methods reported. When comparing Trinitor to existing methods, it becomes evident that conventional approaches typically employ one technology at a time, resulting in a single output and relatively low efficiency in waste heat recovery. In contrast, Trinitor is a polygenerative process that encompasses power generation, cabin heating/cooling, product drying, and oxygen production. By combining two or more low-efficiency systems, Trinitor achieves a notable increase in overall efficiency. Therefore, Trinitor, as a polygenerative process, represents a highly efficient waste heat recovery technology.

Table 1 WHR methods reported

A summary of notable studies on individual components aimed at achieving these goals is presented. Vehicle hybridization improves tactical capabilities by boosting available on-board power while lowering fuel costs [22, 23]. Due to increased power and energy density, as well as less cost, Li-ion battery technology is presently the most often utilized device for electrified systems [24]. Additionally, various energy storage devices can be added to enhance the vehicle’s propulsion power [25]. Proton exchange membrane fuel cell (PEMFC) function at low temperatures, the cooling flow required is higher to compensate for the lower delta temperature between the air and the PEMFC. To help filter the load demand and support the power, fuel cell systems are frequently combined with a lithium-ion battery pack. The world is investigating the use of hydrogen as an alternate fuel. An electrolyzer can generate hydrogen fuel. Automotive Exhaust Thermoelectric Generators (AETEG) unit named “four-TEG system” made up of 240 thermocouples with a power output of 944 W was produced by Wuhan University researchers in 2014 [5]. The output of their TEG is believed to meet the electrical requirements of automobiles [26]. It was put on a military-purpose prototype vehicle called “Warrior” and tested at various speeds, with the findings indicating that the alternator may be replaced with an AETEG unit. Heat storage in vehicles can decrease vehicle setup time for tactical operations while serving as a protection mechanism [27]. Phase Change Materials (PCMs) may require much less mass to achieve the same heat capacity as a conventional heat storage system [302]. Energy density, melting and freezing point, and storage volume are all significantly lower for PCM than for non-phase-change solutions [28]. Thus, the power generated from the engine can be accumulated in a PCM storage tank.

Much research has been published on the subject of reducing diesel consumption and waste heat recovery with cutting-edge technologies. Individually, good work has been done on hybridization, energy storage, TEG utilization, and so on. However, research on the incorporation of a Polygenerative System (Comprising Electricity Production, Produce Drying, Space Cooling/Heating, and Oxygen Production) in automobiles is lacking. This paper is an investigation and selection of components required for the proposed sustainable conceptual polygenerative system model “Trinitor” for automobile applications. The average exhaust gas temperature of V-type diesel engines is in the region of 300–350 °C when they are idle. The Trinitor converts the heat from diesel engine’s exhaust gas into electricity, cooling, and heating. In addition to the foregoing, an electrolyzer can be used to make hydrogen by water electrolysis. This system now includes a capability for supplemental power generation. Hydrogen and oxygen can be generated by the PEMFC using the electrolyzer’s exhaust.

Novelty and objective of the present work

While diesel engine technology has come a long way, a lot of the fuel’s chemical energy is still wasted as waste heat and exhaust. This is roughly the same as 30–40% of the energy that can be obtained from fossil fuels. Exhaust gas temperatures for diesel engines can be anywhere from 250–800 °C, depending on the engine model and the intended use. The entire focus of this work is on selecting the necessary elements for utilizing this thermal energy in automobiles via the proposed sustainable conceptual polygenerative system model “Trinitor” for a variety of purposes such as cabin heating/cooling, oxygen production, power generation, cooking and drinking water purposes. The proposed Sustainable Trinitor model is a novel synergistic combination of numerous processes that collaborate to recover, and reuse waste heat generated by diesel engine combustion in vehicles. To better understand the process, it is divided into four independent units: electricity production, produce drying, space cooling/heating, and oxygen production. The proposed procedure is shown in Fig. 2 as follows:

Fig. 2

Sustainable Trinitor process flow diagram

The simultaneous operation of all four units generates a large amount of electrical and thermal energy. Even with many more technologies across all four units that require electricity to function, this configuration is fairly self-sufficient in meeting demands and storing surplus energy as a backup. This entire process is extremely efficient and sustainable because it does not require any external energy source (other than renewable solar energy) and makes the best use of the waste heat generated. Different components are reviewed in this paper based on their application and usage in the Trinitor system. The proposed procedure has the following advantages:

• Four different types of energy are used to power the vehicle and its sophisticated functions include a reduction in reliance on a single source (Solar PV) and providing backup energy at all times in the event of a problem.

• Cool or hot air can be produced from the cooling/heating chamber depending on demand; this is critical for vehicle comfort. However, cooling/heating is done in a separate chamber; it does not affect the vehicle’s overall efficiency.

• Excess oxygen is stored in the vehicle, which is very important in times of medical emergency and can be accessed via a small bullnose wheel type valve, and bull nose regulator.

• Because the process has no rotating or moving parts (except HAT-Hot Air Turbine), it is entirely supported by thermodynamics, which increases the overall efficiency of the process.

• The use of solar panels in a vehicle takes advantage of sunlight and stores the energy for emergency usage.

• All of the parts are powered by direct current, there is no skin effect on the conductors, allowing the entire cross section of the conductor to be used, resulting in material savings and increased efficiency.

• Additional storage for hydrogen gases can be used as a gaseous fuel for the vehicle’s engine.

• Energy-saving efficiency is a measurement that shows how much product or material is produced in the output from the input energy.

• Greenhouse gas emissions have been reduced.

• Furthermore, it has mobility, scalability, and controllability based on the needs of the end user.

• Under any circumstances, the process can be reversed. (Additional elevation, climatic, and seasonal conditions).

Electricity production

In this section and subsection, the methods used in Trinitor to produce electricity are handled. In this process, basically, heat generated from the engine exhaust is stored in a PCM storage and then is used for turbo-compounding using a hot air turbine. Accordingly, a schematic view of the electricity generation process in Trinitor is illustrated in Fig. 3.

Fig. 3

The process of electricity generation

Engine

Only a small fraction of the energy in exhaust gases is converted into electricity, and the process is permanent because of environmental and other considerations [29]. At temperatures above around 600 °C, exhaust gas emissions exceed 30% of the gas’s energy value, but they decrease with increasing temperature [30]. Under light engine loads, exhaust gas emissions are minimal, but this is crucial for the motor’s peak efficiency [31]. The required application calls for a diesel engine, which has been selected for this prototype model. Table 2 and Fig. 4. show vehicle exhaust temperatures and variables affecting them.

Table 2 Types of vehicles and their temperature change in the exhaust emission [30,31,32]

Fig. 4

Factors affecting exhaust gas temperature

Some researchers have investigated the impact of exhaust backward pressure on diesel engines, for example, on ambient diesel engines in which the pressure difference exceeds the air pressure [33], on 2-stage diesel engines with turbochargers, and turbo-compounding usage [34, 35]. Higher diesel engine exhaust back pressure has been determined to be the cause of increased negative pumping work. The increased pumping effort is neutralized by the current generated by the turbo-compound and also the system’s total thermal effectiveness is improved with the adjusted exhaust manifold pressure.

Filter

In this section, the role of a filter, after the exhaust gases are released is discussed. The types of filters discussed are Diesel particulate Filter (DPF), Wall Flow Filters and Metal fiber filters.

Diesel particulate filter (DPF)

DPF is a ceramic cylinder with thousands of tiny parallel channels that extend in a longitudinal direction toward the exhaust of the engine. The porosity of these filters has been fine-tuned for efficient dust removal with little air resistance [36]. Figure 5 depicts the design parameters of the DPF.

Fig. 5

Design parameters of a diesel particulate filter

DPF substrates

The substrate is the most significant component of a diesel engine filtration system. These filter materials typically collect particulate matter by interception, impaction, and diffusion, as well as holding the particulate matter until regeneration occurs [37]. Since there are many different uses for DPF, from light duty (LD) to medium duty (MD) to heavy duty (HD), the following factors must be considered while designing DPF so that it does not melt or shatter.

• Designing the DPF-Particle Matter (PM) loading and regeneration method must be done depending on individual usage and material limitations.

• Competitive earthing equipment based on its characteristics must be chosen [38]. DPF substrates are shown in Fig. 6.

Fig. 6

Diesel particulate filter and their type of substrates

Wall flow filters

The most popular form of DPF substrate is wall-flow solid ceramic monoliths, developed from catalytic converters. They have a wide surface area per volume unit and a high degree of separation, which sets them apart from other DPF types [36]. Monolithic DPFs usually consist of several minute parallel channels that run axially through the element and are usually square in cross section. The following Fig. 7 depicts the important characteristic features of wall flow filters.

Fig. 7

Characteristic features of wall flow filters [39, 40]

Cordierite and silicon carbide (SiC) are two minerals that are widely used in commercial filters [39]. Cordierite filters are widely known in heavy-duty diesel engine applications [41]. Aluminum titanate is a recent commercial monolith filter material that has been introduced. Figure 8a, b. shows some of the commercial manufacturers and suppliers of these monoliths.

Fig. 8

a Types of wall flow filter substrates, their formula, and commercial producers b the characteristics and applications of the various types of wall flow filter substrates [41, 42]

SiC filter has high thermal efficiency, improved resistance due to thermal shock and thermal conductivity, and higher material strength properties [41, 42]. Due to its high coefficient of thermal expansion, a split design with a filter element is required. It is best suited for our applications which is why SiC filter can be incorporated in the Trinitor prototype [42]. Table 3 shows the characteristics and characteristics of the various types of SiC substrates manufactured by NoTox [43].

Table 3 NoTox SiC model specifications and material properties [43]

Metal fiber filters

The main characteristics of metal fiber filters are shown in Fig. 9. All types of metal fiber composites can achieve up to 85% high porosity. Due to the high porosity of this media, relatively low back pressure is possible for most combinations [44]. Table 4 lists some of the most common commercial metal fiber filters and their parameters.

Fig. 9

Characteristics of metal fiber filters

Table 4 Few commercial metal fiber filters and their specifications [45]

Heat storage

In this section, an analysis of the heat storage capability of different Phase Change Materials has been explored in detail. PCM materials are substances that have a considerably larger range of latent heat storage capacity and, hence are used to store heat.

PCM storage

Thermal energy storage (TES) with phase change materials (PCM) is a simple and practical technique to increase energy storage capacity and consumption in residential and commercial settings [46, 47]. The Trinitor model can make use of the thermal energy storage method of latent heat storage (LHS). Compared to sensible heat storage (SHS), LHS has a higher storage density and a narrower temperature range between heat storage and release, making it an effective way to store heat energy. Recent developments in the design and qualities of revolutionary energy storage materials, particularly micromaterials, have opened up new avenues for improved performance and longevity [48,49,50].

Types of PCM

Potential PCMs for low and high-temperature applications include a wide range of materials, eutectics, and compounds. Figure 10 shows various possible materials that can act as PCM. Tables 5, 6, 7 and 8 depict the thermophysical properties of various PCM types.

Fig. 10

Types of PCM

Table 5 Inorganic PCM compound

Table 6 Types of organic PCM

Table 7 Types of polymers PCM material

Table 8 Types of polyhydroxy alcohols

KClO₄ has the greatest melting temperature of 527 °C and the highest heat of fusion of 1253 J/g among inorganic PCM compounds [51]. Paraffins, fatty acids and their eutectic mixtures, esters, and other organic molecules all contribute to the makeup of organic PCMs. n-Hexaconzane among organic PCMs, has 26 number of carbon molecules, 56.3 °C and 256 J g⁻¹ of melting temperature and heat of fusion, respectively [52].

Polyethylene glycol (PEG) has a melting point, depending on the molecular weight, which ranges from 4–70 °C, with a heat of fusion of 117–174 J g⁻¹. It can be used as PCM because of its high fusion heat due to its crystallinity (83.8–96.4%) [61].

Polyalcohol can also be considered as PCM as it absorbs hydrogen bond energy at low temperatures and the temperature is raised to the solid–solid phase change threshold. For example, the transition phase temperature of neopentyl glycol (NPG) [(CH₃)₂C(CH₂-OH)₂] is 42–44 °C, and the heat of phase change is 110.4–119.1 J g⁻¹ [64, 65].

PCM in the automotive industry

PCMs are used in the automotive industry for engine cooling systems, pre-heating catalytic converters, increasing customer comfort, and combustion engines [66]. Gumus used TES to reduce cold-start discharges from internal combustion engines [67, 68]. Also, during cold start and increasing temperatures stages, pre-heating the engines minimized CO and hydrocarbon pollutants by 64% and 16%, respectively. This explained that PCM stores thermal energy to solve cold start issues in an LPG (Liquified Petroleum Gas)-fueled vehicle. After a waiting period, it was found that EPR (Evaporator and Pressure Regulator) with the addition of PCM to some extent reduces the cold start issue of LPG fuel engines [69, 70]. Heat storage in the accumulator is a unique approach that leads to a reduction in the engine cooling system [70, 71].

Because of their high heat of fusion, various phase change temperatures, little supercooling, the lower vapor pressure in a melt, and chemically inert and stable behavior, paraffin waxes have been widely employed for thermal energy storage [301]. They are also reasonably priced on the commercial market. These PCMs are also non-toxic and environmentally safe [49]. Due to the supporting factors, paraffin waxes can be utilized in the Trinitor prototype. Polyethylene (PE) supports form-stable PCMs because of its chemical affinity for paraffin [72]. Sari made paraffin/HDPE SS-PCMs (solid–solid PCMs) by melt mixing [73]. The greatest percentage of paraffin in the PCM composites for two distinct types of paraffin was 77%, nicely distributed in the solid HDPE matrix. In addition, graphite, expanded and exfoliated by heat treatment, improved the thermal conductivity by 14–24%. Krupa et al. [74] investigated composites of low-density polyethylene (LDPE) and soft and hard Fischer–Tropsch paraffin waxes [75]. Soft paraffin waxes co-crystallized with the LDPE crystals, creating a more compact blend than the Fischer–Tropsch paraffin wax. Macroscopically, the mixes efficient SS-PCMs with LDPE matrix, retaining a compact shape.

Future trends

Over the past two decades, all research articles have shown interest in thermal energy storage using PCM, a fast-emerging topic since 2016. The goal to limit electricity consumption’s ecological impact, especially in pursuit of sustainable growth over the previous decade, may explain this tendency [76]. Nano- and bio-based materials will dominate PCM trends, nanomaterials allow the creation of innovative, high-performance composites. [77, 78]. PCMs for TES also reduce carbon footprints and chemists, materials scientists, and engineers are developing innovative PCMs with improved physical properties, particularly form-stable PCMs. Through improved thermal conductivity, such materials could alleviate the rate problem, reduce storage system size, and simplify production and encapsulation.

Hot air turbine & generator (turbo-compounding)

Turbo-Compounding is a methodology that employs a turbine and a generator in a vehicle to recover the waste heat losses in the exhaust gases released. In the following sections, a thorough study of different types of turbo-compounding implementations and their respective performances has been detailed.

Turbo-compounding (turbine and generator)

Many laws and regulations reduce pollution and fuel waste. 30–40% of fuel is squandered and released into the atmosphere. Thus, vehicle waste heat recovery research and demand have increased. Figure 11 depicts prominent waste recovery methods. This paper focuses on turbo-compounding due to its simplicity, small mass, and low volume, which allows for many real-world applications.

Fig. 11

Waste heat recovery methods

Turbo-compounding is the process of extracting excess waste energy from the engine’s exhaust using a power turbine. An electric turbo-compounding or turbo-generator generates electricity when the power is coupled to the turbine’s output generator [79]. Turbo compounded systems can recover 11.4–25.7% of exhaust energy. Thus, exhaust flow energy recovery becomes 3.7 kW. If linked to an electric generator, the extra power can be used to operate the vehicle’s auxiliary. The combined system generates enough power to run the Trinitor prototype’s lights, air conditioning, and other comforts.

Turbine

Turbo-compounding in diesel engines reduces brake-specific fuel consumption due to turbine efficiency. The tip radius and outlet blade angle of the turbine affect turbo-compound engine performance [80]. Commercial hybrid flow turbines for turbo-compound utilized in 1L turbocharged diesel engines have lower low-pressure ratio capabilities. However, this turbine achieved over 70% efficiency at 50,000 rpm, 1.08 pressure ratio, and 1 kW of output [81, 82]. Radial turbines mismatch exhaust energy pulses in high-pressure turbo-compound diesel engines with split exhaust manifolds. Converting turbine peak capacity to a lower blade-speed ratio improves engine thermal efficiency and exhaust energy consumption [83].

Generator

Most car waste heat recovery power generation systems use High-Speed Permanent Magnet Synchronous (HSPMS) generators. Researchers designed a 10 kW, 70,000 rpm super permanent magnet motor/generator for an electrical turbo-compounding system [84]. HSPM operation and sound in an Electric-Assisted Turbocharged (EAT) system were studied [84]. The slot-less toroidal wound motor outperforms two 6-slot machines of different slot widths in efficiency and noise [84, 85]. A surface-mounted permanent magnet motor for an electrically assisted turbocharger (4 kW and 150 rpm) was studied. According to engine development projects, boosting exhaust system pressure increases energy recovered in the turbo-compound and reduces energy lost to the blow-off pulse when the exhaust valve opens. A negative pressure differential between the inlet and exhaust manifolds increases exhaust manifold pressure and negative pumping effort. Adjusting exhaust manifold pressure improves thermal efficiency and balances pumping labor from the turbo-compound. This diesel engine turbo-compounding study is comprehensive. Waste exhaust heat provides 30–40% of the fuel’s energy. Thus, every waste heat recovery system must recover energy to reduce fuel waste. Turbo-compounding is simple, low-volume, and light compared to other technologies. It also allows engineers to build engines for maximum efficiency. This study found that raising motor-boosted pressure in the turbocharger has improved it by 44.9% compared to the standard turbocharger [86].

Developments in turbo-compounding

Due to its simplicity and compact size, the turbine-generator combination for diesel engine exhaust heat recovery is appealing to many researchers. Power turbine characteristics, two-stage turbine interaction, and steam injection were studied on turbo-compound diesel engine performance. Steam injection and turbo-compound minimize fuel consumption by 6–11.2% at varied speeds [87,88,89]. Power turbine fluid flow, transmission ratio, and turbo-charged turbine fluid flow affect diesel vehicle capacity in a controllable, mechanical turbo-composing system. At 1600 r/min and 970 Nm matching points, the flow of a super-charged turbine 0.75 is estimated. The power turbine flow area coefficient is 1.6, the fixed ratio is 25, and fuel consumption can be reduced by 4.3% and 1.28% [90]. Mechanical turbo-compound has already entered the market and has been adopted by many commercial heavy-duty diesel engine vehicle manufacturers. Table 9 gives us an overview of some of these real-world applications.

Table 9 Mechanical turbo-compounding in heavy-duty diesel engines [91]

Mechanical and electrical turbo-compounding can be improved and developed even when mechanical turbo-compound has become commercial. Table 10 shows new research on turbo-compounding on different engines and its feasibility with other automobile components like turbochargers and compressors to reduce fuel consumption and boost energy production.

Table 10 Recent research work on turbo-compounding

Drying of produce

The heated air from the PCM storage can be used to preserve and maintain hygiene for food products. In this section, we discuss different methods of maintaining food hygiene and preservation using a Hot Air Chamber, also controlling moisture levels using humidifier and dehumidifier.

Hot air chamber

A hot air chamber is a device which will be responsible for keeping food products warm for longer periods using various methods which are discussed in detail in the following sections.

Hot air dryer/chamber

High-temperature water evaporation modifies food’s chemical, physical, and biological qualities simultaneously or sequentially. Shrinkage, textural distortion, discoloration, flavor loss, and surprising texture are the most obvious flaws in dried goods. Food drying degrades rehydration and nutritional quality [100]. (Commercial drying process shown in Fig. 12.)

Fig. 12

Food drying process [101]

Post-collection innovation has used the sun, osmotic, vacuum, hot air, fluid bed, and freeze-drying. Drying entails mass and heat conversion mechanisms that consume energy to remove moisture from food cells [102]. Traditional current drying methods can dry 1 kg of food with 14.53 MJ–90 MJ [103]. Accordingly, it is important to design energy-efficient drying to reduce the food safety cost and carbon impact of conventional electric dryers [104]. Table 11 lists hot air-drying experiments on agricultural products.

Table 11 Studies on agricultural foods using hot air drying

Combination of microwave and hot air drying

During a combination of microwave and hot air drying, convective air movement quickly removes microwave-heated moisture from the product surface without releasing energy into the atmosphere [115, 116]. This approach enhances drying rate, efficiency, and drying time substantially. Mostly, microwave radiation efficiency combines microwave and hot air to control production during drying, and leads to precise heating, which increases the drying rate, drying time, and yield of crops [117].

Combination of infrared and hot air drying

Infrared drying (IRD) uses electromagnetic radiation from 0.78 to 1000 m wavelengths. Infrared radiation is divided into close (0.78–1.40 µm), medium (1.40–3.0 µm), and far (3.0–1000 µm) wavelengths depending on the drying rate [118]. Infrared and hot air-drying together reduce power consumption and drying time, and increase heat and mass transmission [119]. Infrared and hot air-drying crops exposed to infrared radiation enhance molecular vibrations on the crop’s internal surface layers, which accelerates moisture transport from the inner side of the substance. Convection air evaporates water vapor on the material’s surface, lowering its temperature and improving the dry product [120,121,122].

Radio frequency and hot air-drying combination

Qualitative research on the combination of Radio Frequency (RF) (1–300 MHz’s) and hot air-drying showed that, while quality declined, drying costs remained fair [123]. Also, when compared to air drying, there was a reduction in drying time. Energy consumption in this integrated process was only 40% of the individual energy needs [124]. The above Dryers suggest using use cabinet dryer or Infrared dryer in Trinitor. Table 12 lists combined system drying experiments of agricultural goods.

Table 12 Novel heating techniques used for agricultural products

Humidifier

A humidifier is a device that disperses humidity into the air, which may be a requirement for dry climates and rooms with less moisture. In this section, we go through different types of humidifiers and their working in detail. The humidifier receives moist air from the hot air chamber, which is mixed with water from the PEM fuel cell before being transported to the dehumidifier. Chilled air is routed to the chamber from the dehumidifier. A humidifier can be a surface condenser that uses chilled water, or it can be a humidifier that uses tubes to circulate humid air [131], whose types are shown in Table 13.

Table 13 Types of humidifiers

Types of humidifiers

Table 13 summarizes the types of humidifiers, working principles, advantages, and disadvantages.

Why we choose a revolving wicks type humidifier?

Evaporative humidifier wicks collect minerals and other impurities in water. The wick keeps this chemical from entering your air. Evaporative humidifiers improve allergy and asthma symptoms in medical trials. The wick humidifier absorbs water from the force and dematerializes over a broader facial area. The addict blows air across the wick to evaporate water. Moisture affects wick evaporation. This humidifier automatically reduces water vapor production when room humidity rises.

Applications of humidifier

Music rooms, museums, and galleries can employ humidifiers to preserve paintings and instruments. Public and industrial buildings use air humidification to maintain humidity. Humidifiers are needed in freezers to prevent food from drying out. Static difficulties affect packaging, printing, paper, plastics, textiles, electronics, vehicle manufacturing, and medications. Friction can generate static buildup and sparks at RH below 45%. Static arises between 45 and 55% RH, but never above 55%.

Dehumidifier

A dehumidifier is a device that reduces humidity from the air, which may be a requirement for moisturized climates and rooms with higher moisture levels. In this section, we go through different types of dehumidifiers and their working in detail.

Dehumidifier

Dehumidifiers lower and maintain air humidity. This is done for health, thermal comfort, musty odor removal, and mildew prevention by extracting water from the air. Dehumidifiers should lower air moisture to 30–50% relative humidity [132]. Figure 13 shows a dehumidifier enthalpy diagram. A dehumidifier enthalpy graph visually depicts how the enthalpy of air changes during dehumidification. It shows the transformation of incoming humid air into drier air, with lower enthalpy values. This graph is essential for assessing the dehumidifier’s efficiency and understanding how air properties change in the process.

Fig. 13

Enthalpy diagram for dehumidification

Need of dehumidifier

RH is set in the dehumidifier’s control system and the control system switches off when RH reaches the desired level. When the environment reaches a pre-set RH, the device restarts, repeating the cycle to ensure that your environment is never below prescribed RH values and that it is not wasting energy by running continuously [133]. Dehumidifiers maintain airflow by blowing dry, conditioned air to where it’s needed, eliminating the need for ventilation.

Cooling coil type dehumidifier

This type dehumidifiers fit our Trinitor and a fan-assisted filter removes room air. An evaporator coil cools the filtered air below its dew point. Dehumidifying air condenses when the temperature drops below the dew point [134]. The condensed vent forces retrieved condensed water. The dehumidifier’s humidistat measures humidity [135]. Dehumidifier selection is shown in Table 14.

Table 14 Application suitability table

Humidification—Dehumidification

Humidification–Dehumidification (H/D) is a process that produces freshwater by loading a carrier gas, usually air, with water vapors till saturation. H/D uses forced fluid circulation to humidify and dehumidify in two chambers. [136].

Applications

Pharmaceutical, food and drink, cold storage, waterworks and utilities, lithium battery manufacture, nuclear, automotive, aviation, chemical processing, car storage, archives, and wind farms use dehumidifiers. A dehumidifier, in essence, is an effective solution for practically any place that requires humidity control [133].

Space cooling/heating

After stabilizing the charged controller, the addition of a Photovoltaic Cell (PV) that catches solar radiation helps to maximize the potential for generating and storing extra electrical energy. The device thermo-electric generator (TEG) makes use of the “Seebeck’s Effect concept” (TEG). According to this theory, the temperature difference between two dissimilar electrical conductors or semiconductors causes a voltage difference, which produces electricity. TEG is a collection of similar semiconductors connected in series to maximize overall output current. Following humidifier treatment, the chilled air from the Cooling chamber delivers the required temperature reduction. TEG converts heat energy to electrical energy, which can be used immediately or saved for use in future circumstances. Furthermore, we can induce temperature differences by giving the requisite power, which is the inverse of Seebeck’s effect (also known as Peltier’s Effect).

PV/T Panel

A photovoltaic-thermal solar panel is responsible for capturing both electricity and heat from solar energy. We use a PV/T panel in Trinitor as it provides another source of power and the heat captured can be stored in the Latent heat storage. In this section, we discuss many types of PV/T panels, their applications, performance, and work in much detail.

Solar PV/T panel

Renewable energy sources currently meet 13.5% of the world’s key energy demand, with great future potential [137]. Different methodologies are used in the current study to reveal the classifications of PV/T hybrid solar collector systems. It also includes the primary applications used in the system under review. New technologies, such as the integration of TEGs into the PV/T system, are also being tested. The center of the effort is depicted in Fig. 14.

Fig. 14

Work focus

Types of PV/T collector

Figure 15 depicts the many types of PV/T collectors. Flat plate PV/T collectors have the same appearance as flat plate thermal collectors. The only obvious difference is the PV panel mounted on the upper side of the metal absorbent plate [138]. Focused or concentrated PV/T collectors use concentrators to improve the amount of irradiance reaching PV modules. Brogen et al. investigated the cold water-focused PV/T collector for the integrated structural type. PV/T series modules are used in conjunction with low-cost aluminum foil screens that have a dual concentration of 4.3 [139]. Coventry invented the “CHAPS” (combined heat and power solar) PV/T collector, consisting of monocrystalline silicon cells, with a two-axis tracking system and a parabolic trough with a concentration ratio of 37 [76]. To capture the majority of the heat produced, a water pipe and ice breaker were attached to the back of the cells.

Fig. 15

Types of PVT collector

Water type PV/T collectors are classed based on water flow pattern. They are classified as sheet and tube, channel, free flow, and two forms of stretch [140]. The air flow pattern also reflects the air conditioning PV/T collector.

Water type PV/T system

Water type PV/T system uses water as the working fluid medium for its operation. We discuss, the performances and utility of different types of PV/T – water systems.

Hendry and Raghuraman thoroughly examined the performance of the hybrid PV/T collector [141]. The tests are conducted outdoors, with varying quantities of intake fluid and weather circumstances, the thermal losses and efficiency coefficients obtained was 6.77 W mK⁻¹ and 0.62, respectively [142]. Lalovic created and tested an amorphous silicon PV/T collector (a-Si). The overall surface area of the Si-PV cells employed in the investigation was 0. m² and the efficiency was average. According to test results, the hybrid PV/T collector performs better as a heat collector, heating water to a temperature of 65 °C. On the other hand, the system’s electrical performance hasn't changed significantly. In early 2010, many researchers sought to create hybrid PV / T water collectors [143, 144]. systems. Table 15 describes many types of PVT-water systems. PVT-water with 14% electrical capacity and 60% thermal efficiency will be suitable for Trinitor process.

Table 15 Types of PVT-water systems

Thermoelectric generators in PVT system

Photovoltaic thermal systems use TEGs to increase solar energy use. TEG generates electricity via the Seebeck effect and improves the electrical system conversion efficiency. This method works better for rooted systems since it provides more electricity at greater temperatures [149]. TEGs’ no-moving-parts and compactness are advantages. The autonomous systems create far less energy [162]. The basic architecture is PV modules with TEG modules attached as illustrated in Fig. 16. The TEG module’s cool side has a finned heat sink that cools the TEG and PV.

Fig. 16

Schematic diagram of PVT-TEG

The advantages of TEGs are numerous like:

• Direct energy conversion, unlike heat engines that convert thermal energy into mechanical energy and employ an alternator to generate electricity.

• Maintenance is unnecessary because the TEG has no working fluids or moving parts.

• A prolonged life, particularly when working with continuous heat sources.

• Quiet operations.

Future trends of PV/T collectors

Table 16 shows some likely future trends in some markets for PV/T systems, which could become crucial to the renewable energy industry worldwide [164]. To attain the goals, more research is needed.

Table 16 Possible future market segments of PVT

Charge controller

A charge controller is a device that is used to regulate the voltage and current from the PV panel array to the battery. It is responsible for preventing overcharging and excess discharging from the battery. In this section, we discuss the types of charge controllers and their performance characteristics. A schematic understanding of the charge controller is shown in Fig. 17.

Fig. 17

Solar charge controller functions and types

Modern high-performance battery chargers use PWM charge controllers (Fig. 18). PWM pulsing has various advantages, including 1. Locating and de-sulfating a misplaced battery, 2. Dramatically boost battery charge acceptance, 3. Maximum battery capacity, 4. Equalize wandering battery cells, 5. Reduce heat and gas emissions.

Fig. 18

Solar PWM charge controller’s diagram

Eftichios Koutroulis et al. developed a new MPPT system with a microprocessor-controlled buck DC/DC converter [163]. The recommended MPPT system used PV output power to directly regulate the DC/DC converter, simplifying the system. The system is more efficient, cheaper, and adaptable to varied power sources (e.g., wind generators). The MPPT controller increases PV output power by 15% compared to DC/DC conversion cycles to reach a peak energy of 1 kW m and 25 °C [164].

MPPT will detect high solar radiation entering the PV module and create high power. As a result, it generates standard system costs [165]. In some circumstances, MPPT charge control is employed to dispose of the PV module’s energy to form a’high power point’ [165]. Tables 17 and 18 compare PWM and MPPT pros and cons. Trinitor can use only MPPT charge controllers to boost PV/T system output by 30%.

Table 17 Advantages of PWM and MPPT

Table 18 Disadvantages of PWM and MPPT

Battery

In this section, a thorough review on different types of batteries suitable for various applications are discussed along with their applications and performance. The kind of battery used may differ depending on whether the vehicle is powered by an All-Electric Vehicle (AEV) or a Plug-in Hybrid Electric Vehicle (PHEV). Present battery technology aims for 8 years lifespan [169]. Some batteries endure 12–15 years in a chilly atmosphere and 8–12 years in a solid environment. Lithium-ion, nickel-metal hydride, lead-acid, and ultra-capacitors are batteries used in electric vehicles (Fig. 19). Due to their high-density, high-power durability, longevity, and environmental friendliness, lithium-ion type batteries are suitable for Trinitor (Fig. 20) [170]. The advantages and disadvantages of batteries are displayed in Table 19 and Fig. 21a–d [171].

Fig. 19

Types of battery

Fig. 20

Types of Li-ion battery

Table 19 Advantages and disadvantages of different batteries

Fig. 21

a Battery versus specific energy b Battery versus energy density c Battery versus power density d Life cycle

Thermo-electric generator

Thermo-Electric Generator is a device that generates electricity whenever, there is a temperature difference between the ends of a semiconductor. This principle is known as the Seebeck’s Effect. The converse of it is also true, i.e., if electricity is flowing through a semiconductor, there will be temperature difference created. In this section, various types of materials for TEGs, their utility and applications are explored.

Thermo-electric generator

Vehicle waste detection employing TEGs has grown in popularity [176,177,178]. TEG converts thermal energy into electricity using Seebeck’s effect and is compact and low-maintenance. Vehicle waste disposal methods like Organic Rankine or the turbocharger cycle are ineffective. TEGs don’t have moving components or liquid like refrigerators, therefore they can handle this power with minimum vehicle performance disturbances.

Material researches of thermoelectric devices

Figure 22 shows the TEG material category.

Fig. 22

TEG materials

Semiconductor

Materials in Fig. 22 make TEG. Semiconductor materials (Seebeck coefficient above 100 V/°C) reduce thermal conductivity (k) without changing thermal diffusivity (α) on numerous objects, increasing Figure of Merit (ZT). Ilanga et al. examined and discussed organic n-type semiconductors for thermoelectric devices and concluded ZT is best for small bandgap semiconductors [179]. Some intermetallic compositions, including Mg₂X (X = Si, Ge, Sn) (ZT of Mg₂Si at 682 K = 0.86), have higher specifications like high coefficient of Seebeck, low electrical resistance, and low thermal performance. Anti-fluorite composition boosts performance and thermoelectricity [180]. Another strength is their anti-fluorite composition for high performance and effective thermoelectric properties.

Ceramic

Metal oxides have better chemical stability, resistance to oxidation, low toxicity, and low cost than Te alloys, enabling longer-lasting electronics [181]. Ceramic, a thermoelectric material, can be used in combustion engines and incinerators for heat recovery [182]. Due to their low carrier mobility, they were not good TE materials before Na_xCo₂O₄ oxides. TE units with good TE characteristics have used cobalt oxide and cadmium oxide as p- and n-type semiconductors, respectively. Nonstoichiometric CdO has acceptable electrical conductivity. High valence elements and matching dopants reduced its resistivity [183]. Sb₂O₅ dopant on n-type SnO₂ increases electrical conductivity and has similar carrier mobility to oxide [184]. Wang et al. examined Cd_1-xPr_xO ceramic thermoelectric properties at higher temperatures. CdO doped with 0.1% Pr has 0.380 ZT at 1000 K [181, 185]. Zhu et al. examined TE performance after doping CaMnO₃ with lanthanides and other rare-earth elements [181]. The optimized dopant’s ZT is 0.200 after replacing Yb or Dy. Double doping increases ZT substantially.

Polymer

Toxic compounds, natural resource restrictions, and high-tech, expensive production methods limit inorganic thermoelectric materials [186]. The protective polymer matrix and conductive filler are safer and more ecologically friendly than previous thermoelectric modules for separating polymeric conductive compounds. Thus, mechanical flexibility, cheaper manufacture, solution process area, and lightweight were examined in these synthetic materials [187]. Elmoghani et al. shown that polymers can power thermoelectric machines with human body heat [188]. Lu et al. examined ways to improve n-type polymers in thermoelectric devices [189]. Pang et al. examined the thermo-electric performance of a continuous polymer compound with CNTs and bismuth telluride, specifically a ZT-related value [190]. Table 20 shows that the most extensively utilized Bi-TE products are promising TE materials. This table indicates that ZT is closer to unity at room temperature, which is bad.

Table 20 ZT of the Bi–Te-based material

Application

The automobile sector has focused on thermoelectric generators, which transform outbound electricity in IC engines [189]. Fuel-efficient vehicles are used for gas extraction 40% and motor vehicles and equipment 25.5% [205]. Due to the wide variety of IC engines, different thermo-elements are needed to increase conversion efficiency. These applications utilize sub-thermoelectric materials due of their wide temperature range. Split TE materials include N- and P-type Bi₂Te₃ at low temperatures (250 °C), PTAGS and NPbTe at medium temperatures (250–500.0 °C), and Schuterite Materials (PCeFe₃RuSb) and (NCoSb₃) at higher temperatures (500–700 °C). The authors created a split material TE configuration to track segment thickness, thermal expansion coefficient, and module energy efficiency. This design uses flat TC solutions with a TE item between heat source and heat sink. Automotive applications use split TEGs and TEG cascades. This system’s unique mechanical structure prevents partition instability. Willebrecht and Beatles Schmidt introduced two train car cascades in which Bi₂Te₃ (225 °C) and Mg₂SiSn / MnSi (415 °C) power the 2.55 kW cascade TEG [206]. IC engine manuals employ two TEG sites: radiator and exhaust heat exchanger. Crane et al. showed that TEG radiator integrated cooling system can absorb enough energy to power the alternator [162]. Ceramic substrates are suitable for Trinitor TEG to decrease heat transmission between TC arms and achieve substantial temperature differences between hot and cool surfaces.

Oxygen production

Oxygen production is done with the atmospheric air drawn in by the dehumidifier fan. Hot drying chamber exhaust is low temperature high moisturized air with moisture content of 60–70%. Water, a byproduct of PEM fuel cell, is sprayed over the flowing moist air stream making it highly humidified and saturated, which is dehumidified. The chilled air is combined with the moist air from TEG to enhance cooling effects. The collected water can be used as potable drinking water and excess is routed to an electrolyzer. An electrolyzer separates hydrogen and oxygen gases by passing electricity through water, later they are stored. The oxygen can be stored in separate gas balloons or pockets and used as an oxygenator for civilians when needed. When necessary, the H₂ and O₂ gases can be passed through the PEM fuel cell to produce electricity and water for the humidifier. After being stabilized and controlled by the charge controller, this electricity is stored in the battery.

PEM fuel cell

Proton Exchange Membrane Fuel cells are responsible for producing electricity using hydrogen and oxygen gases as intakes with water as by-product. The utility of this in the Trinitor is to generate power in vehicle using stored oxygen and hydrogen gas. In this section, the performance, application and working of PEMFC is discussed in detail.

PEM fuel cell

PEM fuel cells’ adjustable design and low operating temperatures have made them popular as passenger vehicle and stationary power sources [207]. It generates electricity from fuel electrochemically like a battery [208]. The amount of fuel available for conversion to energy in a battery is limited, however with a PEM fuel cell, fuel is continually delivered into a system from an external source. One significant fuel cell type that can run at lower temperatures varying from − 40 to 115 °C is polymeric membrane fuel cells [209].

Composition of proton-exchange membrane fuel cells

Figure 23 and Table 21 show PEM fuel cell components and specs. Table 22 displays the comparison of proton conductivity and PEMFC performance on Nafion-based composite membranes. Figure 24 shows the requirements to be fulfilled by PEM fuel cell for gasket.

Fig. 23

Components in PEM fuel cell

Table 21 PEM component specifications

Table 22 Comparison of proton conductivity and PEMFC performance on Nafion-based composite membranes

Fig. 24

PEMFC gasket requirements gasket

Recently, poly-benzimidazole (PBI) membranes with a glass transition of 420 ℃ have shown promise for high-temperature functioning due to their thermal stability [231]. PBI and phosphoric acid-doped PBI (PA-PBI) membranes offer excellent proton conductivity, low gas permeability, low electro-osmotic drag, and excellent oxidative and thermal stability [232]. As PA doping and operation temperature increase, mechanical stability decreases, whereas PBI molecular weight increases [233, 234].

Working process of pem fuel cells

Figure 25 shows the working process of PEMFC.

Fig. 25

PEMFC working process

Application

Fuel cells are gaining popularity as pollution and carbon emissions decrease. PEMFCs are best for transportation. Wheelchairs, e-scooters, motorbikes, and wagons use proton-exchange membrane fuel cells [235], Fig. 26 lists a few uses.

Fig. 26

Application of PEM fuel cell

Cold startup

Automobile fuel cells also need cold-start capability, as fuel cell vehicles must start in sub-zero conditions. When fuel cell temperatures drop below 0 °C, the fluid inside this electrode freezes and fills the vacuum space with ice. A hybrid battery may warm the stack above 0 °C to expel water by capillary action via liquid flow, making self-starting more practical, reliable, and attractive. A potential 10 s cold-start under 0.10 A cm from 30 °C requires a heat capacity to membrane area ratio of 300 J K⁻¹m [236].

Electrolyzer

An electrolyzer is a device responsible for breaking a water molecule into oxygen and hydrogen respectively. In our Trinitor, the purpose of an electrolyzer, is to generate oxygen for medical requirements. In this section, we discuss different types of electrolyzers, their performance and their utility in detail.

Electrolyzer

Electrolyzers supply PEM fuel cell with H₂ and O₂. PVT modules/batteries power hydrogen electrolyzers. After compressing and storing H₂ in a gas tank, air is filtered and sent to the PEMFC stack to make O₂. Nonhomogeneous temperature changes may produce electrochemical response, reducing PEMFC lifetime, which is avoided by maintaining PEMFC working temperatures [237, 238]. A PVT module or battery generates electricity since the electrolyzer and fuel cell cannot consume and create power simultaneously (charge and discharge mode).

Types of electrolyzer

PEM, alkaline, and SOEC electrolyzers are the three primary types (Table 23). Electrolyzers work differently depending on the electrolyte. PEM Electrolyzer is ideal for Trinitor.

Table 23 Types of electrolyzer

PEM electrolyzer

PEM electrolysis and PEM fuel cell technology use solid poly-sulfonated membranes (Nafion) as electrolytes [244]. Proton exchange membranes limit gas permeation, exhibit strong proton conductivity (0.10 ± 0.020 S cm⁻¹), low thickness (20.0–300 m), and rise under pressure [245]. PEM electrolyzers use electro catalysts like Pt/Pd for Hydrogen Evolution Reactions (HER) at the cathode and IrO2/RuO2 for Oxygen Evolution Reactions (OER) at the anode, making them more expensive than alkaline water electrolysis [245].

Components of electrolyzer

MEAs, current collectors, and separator plates make up a PEM electrolyzer. MEA splits the cell in half as the electrolyzer anode and cathode. IrO₂ powder was used to thermally treat the oxygen evolutionary anodic catalysis. Because of its high flux density (2.00 A cm), durability, proton conductivities, and mechanical strength, Nafion 115 membranes are used as solid polymer electrolytes [242]. Table 24 shows PEM fuel cells with varying cathode loadings. Current collectors are essential to the PEM electrolyzer’s process and cell efficiency. Titanium plates are employed as potential current collectors in PEM electrolysis of water systems because to its electrical conductivity, structural robustness, and acid resistance. Bipolar plates and gaskets surround porous titanium plates that act as current collectors and gas diffusion layers (GDLs) on both sides of the MEA. Current collectors allowed electrical current to flow between the electrode and bipolar plates [246]. Titanium grid systems, carbon current collectors, and stainless-steel grids have inferior electrochemical characteristics than pure titanium plates. PEM electrolyzer separator plates are currently stainless steel, titanium, and graphite. Many electrolyzer systems use different separator plate designs to improve performance, but a horizontal straight flow field has shown promising electrochemical activity, particularly in PEM electrolyzers [247].

Table 24 Summary of full PEM Electrolyzer with a cathode

Challenges of electrolyzer

Thermally generated IrO_x has higher stability but lesser activity than electrochemically manufactured oxides or hydrous IrO_x. A rising number of experiments on relatively long electrolysis operations lasting hundreds or even thousands of hours have already been published in the recent literature. At a reduced current load of 1.0 A cm², IrO_x sustained on a Ti catalyst containing 50% Ir and then a low catalyst concentration of 0.125  cm² operated for more than 1 thousand hours [258]. The highest reported stability was achieved with nano—structured thin film (NSTF) cell, that attained 5 thousand hours with the same current load of 2.0 A cm² and an Ir loading of 0.255 mg cm² [259]. No other supporting catalyst has achieved the same level of catalytic durability as titanium-assisted catalyst [260].

Application of electrolyzer

The PEM electrolyzer is a low-carbon energy and chemical storage solution for enterprises. Hydrogen is an attractive storage device for surplus sustainable power because it can be used for energy production during peak demand periods, oil and gas grid supplementation for efficiency improvements, transportation fueling, and chemical biofuel production for green fertilizer and other chemicals. [261]. The Polymer Electrolyte Membrane (PEM) Electrolyzer, excellent for large-scale hydrogen generation, uses an ionically conductive solid polymer. [262].

Analysis of trinitor

Advantages and limitations of each technique

Electricity generation

Turbo-compounding provides numerous advantages. It enables the recovery of waste heat from engine exhaust, leading to a reduction in CO₂ emissions and enhanced fuel efficiency. Approximately 30% of exhaust gas can be recycled as a power source, thereby improving overall efficiency [263]. Moreover, turbo-compounding has the potential to boost engine mechanical power, with research indicating an increase of up to 18% in power output. It can also generate additional electrical power from engine exhaust energy, with potential gains of up to 1.1 kW. In summary, turbo-compounding presents opportunities for enhanced performance, lowered emissions, and increased fuel efficiency in internal combustion engines. Turbo-compounding does have its limitations, particularly in terms of power losses and increased pumping work. The power turbine, which is situated in series with the main turbine, experiences power losses due to higher back pressure, leading to increased pumping losses [264]. Furthermore, in turbo-compounding setups within passenger cars, the utilization of waste heat is limited since only a small portion of the exhaust energy is required for compressing the intake air.

MPPT charge controllers provide various advantages within PVT systems. They excel at extracting more electricity from solar panels by operating at the panel’s maximum power voltage, resulting in improved charging efficiency and increased power output. These controllers effectively manage battery charging, ensuring it’s done correctly and preventing overcharging. MPPT controllers outperform PWM controllers, boasting higher efficiency levels ranging from 94 to 99% [265]. Overall, the incorporation of MPPT charge controllers in PVT systems enhances battery charging regulation, boosts efficiency, and maximizes power extraction from solar panels. However, it’s essential to note that unchecked energy consumption by electrical equipment can lead to system failures and a reduced system lifespan when using MPPT charge controllers [266].

Drying of produce

Both hot air and infrared food drying methods offer several advantages. Infrared radiation, when applied carefully, can enhance drying kinetics, resulting in faster drying at elevated temperatures and reduced distances. However, it is crucial to avoid extreme values to prevent overheating. The utilization of infrared radiation can also enhance food quality by lowering water activity, minimizing color changes, and retaining essential nutrients. In numerous studies, infrared drying has proven to be more efficient and capable of producing higher-quality products compared to conventional drying techniques. Furthermore, the combination of hot air and infrared heating can deliver uniform heating, reduce drying duration, and enhance energy efficiency [267]. In a hot air-infrared system, maintaining a minimum hot air velocity of 0.3 m/s is advisable to ensure proper control of product quality. Overall, both hot air and infrared drying methods have their respective advantages and disadvantages, and the choice should be based on the specific requirements of the drying process [268].

Space cooling/heating

The advantages of Peltier and Seebeck effect cooling/heating systems encompass their utilization of green infrastructure, absence of harmful gases, straightforward design and construction, compact dimensions, and eco-friendliness. These systems do not rely on moving components like compressors or solution pumps, reducing the risk of mechanical failures. Moreover, they have the potential to mitigate the adverse impacts associated with traditional refrigeration systems, including issues such as skin cancer, ozone depletion, and contributions to global warming. Additionally, cooling/heating modules based on the Peltier effect can be fine-tuned for optimal coefficient of performance and cooling/heating capacity, thus enhancing their overall efficiency. Notably, Peltier effect cooling systems have been effectively deployed in various applications, such as air conditioning and emergency cooling for electric vehicle batteries, ensuring user safety and comfort. Nonetheless, Peltier effect cooling does have limitations, including relatively modest cooling performance when compared to conventional refrigeration. Another constraint involves the necessity to optimize the power supply current to achieve the desired cooling/heating capacity and coefficient of performance [269].

Oxygen generation

The utilization of PEM electrolyzers for oxygen generation offers several advantages. Firstly, it enables highly efficient electrodes that are straightforward to manufacture and cost-effective. Secondly, PEM electrolyzers are adaptable and can function with both vapor and liquid feed systems, providing versatility for various applications. Moreover, these electrolyzers play a crucial role in the production of renewable hydrogen, which is essential for achieving decarbonization and sustainability goals [270]. However, the commercialization of low-temperature fuel cells and electrolyzers faces challenges related to stability. Addressing these challenges necessitates the exploration of novel approaches for assessing degradation. Factors such as electrode architecture, the nature of the electrolyte, reactant and product transport, and operating conditions should all be considered in this context [271].

Overall performance

The polygenerative process typically provides greater efficiency compared to the individual efficiencies of the processes it involves. Polygenerative systems often involve synergistic interactions between processes, where the output of one process complements or enhances the performance of another. This cooperation between processes can lead to higher overall efficiency than if they were operated independently. Thus, theoretically with literature support trinitor can achieve overall efficiency of 40.12–54.81%. The actual improvement in efficiency may vary slightly depending on the system’s design and components; nonetheless, the underlying thermodynamic concepts support the theoretically achieved efficiency range with literature support. An article with empirical results from real-time applications and case studies will be published soon. When temperatures exceed 600 °C, the Trinitor excels in capturing waste heat, recovering nearly 40–45% of the energy from exhaust gases [272]. This highlights its effectiveness in capturing energy that would have otherwise been squandered, helping to promote more sustainable energy use [273]. Trinitor dramatically reduces exhaust gas emissions while an engine runs at a lower load. This decrease is crucial since it coincides with a motor’s peak efficiency and emphasizes the Trinitor’s reduction in emission’s beneficial effects on the environment [274]. Table 25 lists the performance of individual components used in Trinitor reported in other articles.

Table 25 Performance of individual components reported in other articles

Conclusions

A comprehensive review of all the techniques, principles, chemicals, components, etc. available for exhaust waste heat recovery forms the basis of this paper’s investigation into, and selection of, the necessary components for the proposed sustainable conceptual polygenerative system model “Trinitor” for diesel vehicles. According to the findings of the review:

• The use of SiC wall flow-Diesel Particulate Filters (DPF), a paraffin-based Latent Heat Storage (LHS) System, and electric-assisted turbo compounding allows the electricity production unit to operate efficiently at minimal cost.

• The produce drying unit can work efficiently and cheaply by using hot air drying or infrared drying, a revolving wicks humidifier, and a cooling coil type dehumidifier.

• The space cooling/heating unit requires water type PV/T collector, Maximum Power Point Tracking (MPPT) charge controller, Lithium-ion batteries and ceramic-based TEGs for efficient output.

• A (Proton Exchange Membrane) PEM electrolyzer with appropriate components (bipolar plates, electrodes, catalyst, membrane, and gasket) contributes to the efficient operation of the oxygen production unit.

Future scope

• A future publication will present an energy, exergy, economic, and environmental analysis of the Trinitor prototype, constructed using the components identified in this review.

• The challenges that could arise during the implementation of Trinitor, along with their respective solutions, have been presented:

• When moving critical parts and equipment to difficult locations, logistical issues may occur. When building successful transit and setup solutions, we must consider transportation methods, assembly requirements, and on-site help. Selecting the optimal locations for storing important parts near the deployment area allows for faster assembly. This saves time on transportation [290].

• On a big scale in remote locations, the Trinitor setup may lack the necessary supporting infrastructure. Thoroughly analyze the location and collaborate with local officials to ensure that appropriate infrastructure is available or can be created. Simplifying shipping and on-site assembly, use modular designs and smart logistics planning can be done to overcome the challenge [291].

• The cost of implementing a large-scale Trinitor system in certain locations can be a significant barrier. To address this, consider exploring funding options, grants, or partnerships that can help offset costs and make the technology economically viable for deployment in remote areas [32].

• Ensure that the supplies and equipment required for a substantial Trinitor installation are readily accessible or can be obtained in a sustainable manner. Exploring alternative materials and implementing efficient resource management techniques can help reduce costs and optimize resource utilization [292, 293].

• Ensuring a reliable hot gas or air source in remote or hard-to-access areas can be a challenge. To reduce dependence on a single energy source, it is essential to conduct a thorough analysis of the location’s solar radiation patterns and explore potential alternative energy sources. One solution is to implement hybrid energy systems that combine solar, biomass, or other renewable energy sources to ensure a consistent energy supply [294].

• Even though, the proposed application for this process is focused on mobility and small-scale heavy-duty vehicles specifically, this process can also be applied on a large scale and a stationary plant-like setup can be done in very inaccessible locations to satisfy basic power, water needs and temperature modulation of products. When scaling up the Trinitor process for larger stationary plant-like configurations, there may be challenges in maintaining efficiency and optimal performance. To address this, rigorous engineering and modeling studies should be conducted to optimize the system design. Consider factors such as heat distribution, material constraints, and the seamless integration of components to ensure sustained performance [295].

• Maintaining consistent and reliable performance of the Trinitor system, particularly in fluctuating environmental conditions, poses a significant challenge. To address this, conduct simulations and tests should be in diverse environments to ensure the system’s dependability and resilience. Redundancy techniques and fail-safes should be implemented to prevent performance interruptions and ensure continuous energy production [296, 297].

• The development and adoption of Trinitor technology may be hampered by complex and ever-changing regulatory frameworks related to renewable energy. Participating in policy advocacy and collaborating with government agencies to assist create favorable policies that promote the usage of renewable energy. Collaborating with legal specialists to streamline the Trinitor deployment approval process and ensure compliance with current legislation. Joining trade associations to influence policy debates and establish rules that will benefit your industry will help overcome regulatory challenges [298].

Feasibility and practicality

This section discusses the viability and practicability of introducing the Trinitor system, with a particular emphasis on cost-effectiveness, maintenance requirements, widespread acceptance, and compatibility with existing vehicle systems.

• As per a research report evaluation, the initial installation of a Trinitor system in automobiles can incur substantial costs. This technology may prove prohibitively expensive for certain applications, particularly when considering the high cost of system components and vehicle integration. However, as a polygenerative system, Trinitor effectively minimizes expenses associated with multiple outputs. Consequently, it can achieve a swift payback period and a prolonged operational lifespan, ultimately resulting in cost-effective operations for the majority of its lifecycle. The recommended approach for cost effectiveness involves exploring government subsidies, tax rebates, or incentives to offset initial expenses. Enhancing the efficiency of the supply chain and collaborating with dependable and economical suppliers can lower material and logistical costs, ultimately enhancing overall cost-effectiveness. Negotiating advantageous contracts and simplifying procurement procedures are crucial components. Furthermore, continuous research and development efforts to reduce manufacturing costs of Trinitor components and enhance productivity can further contribute to increased cost-effectiveness [299].

• Maintenance of a polygenerative system used for waste heat recovery is essential to ensure that it continues to provide energy savings and environmental benefits. Due to the limited number of moving components in the Trinitor system, its maintenance requirements are minimal. Implementing fundamental maintenance procedures, including routine inspections, cleaning, sensor calibration, performance monitoring, energy audits, optimization, adherence to safety protocols, maintaining a spare parts inventory, and ensuring environmental compliance, is essential for prolonging the system’s longevity. However, specialized maintenance and a workforce with the necessary expertise can potentially facilitate broader adoption. The Trinitor system, whether applied in a power plant or as part of a vehicle, requires regular maintenance to ensure peak performance. Overcoming the maintenance challenge requires the development of user-friendly maintenance protocols and training programs, including digital training methods. Collaboration with local service providers or offering incentives for training can help guarantee the availability of a skilled workforce for system maintenance. Constructing the Trinitor system to withstand extreme environmental conditions and utilizing high-quality, durable materials can effectively minimize maintenance needs and extend the system’s operational lifespan. Enhanced durability results in reduced frequency of replacements and repairs, leading to cost savings and improved overall performance [14].

• The integration of Trinitor systems may pose mainly space, compatibility, mass and balance challenges, especially with older vehicles. Adapting the Trinitor model to operate effectively across different car makes and models can be a complex task. To address this, designing new vehicle models in collaboration with automobile manufacturers, which feature integrated Trinitor systems, will ensure compatibility and optimal performance. Additionally, to promote wider adoption, exploring retrofitting options for older vehicles should be considered [6].

• Enhancing the adoption of Trinitor technology may require bolstering its credibility. The public might require assistance in understanding the benefits of this technology or may have concerns regarding its efficiency and safety. To mitigate these concerns, public awareness campaigns, educational programs, and the presentation of successful case studies can play a pivotal role. Collaborating with educational institutions and media for informative campaigns can positively influence public acceptance. Additionally, real-world examples of vehicles on the road utilizing Trinitor technology can significantly bolster public confidence in its capabilities and safety.