Energy and Exergy Analyses of a Combined Multigeneration System

Abstract

A renewable energy-based system is proposed in the present study which will produce electricity, hot water, space heating and cooling, fresh water, and hydrogen. Thermodynamic modeling including energy and exergy analyses is performed to evaluate the feasibility of the system under the proposed operating conditions, and parametric study is performed to assess the system performance for varying conditions. The performance of the baseline system results in energy and exergy efficiencies of 0.28 and 0.25, and for the multigeneration system, efficiency values of 0.69 and 0.54 are achieved. In an additional study using actual Canadian offshore wind speed data and ocean current conditions, the multigeneration efficiencies for the present system range between the values of 0.32–0.48 energetically and 0.22–0.35 exergetically.

Nomenclature

A :

Area, m²

ACH :

Air changes per hour

c _p :

Specific heat, kJ/kgċK

COP _i :

Coefficient of performance (i = H for space heating, i = C for cooling)

CP :

Power coefficient

d :

Rotor diameter, m

ex:

Specific exergy, kJ/kg

$\overset{.}{ Ex}$ :

Exergy rate, kW

h :

Specific enthalpy, kJ/kg

$\dot{H}$ :

Enthalpy rate, kW

$K\dot{E}$ :

Kinetic energy, kW

$\dot{m}$ :

Mass flow rate, kg/s

p :

Pressure, kPa

P :

Power, kW

$\dot{Q}$ :

Heat rate, kW

T :

Temperature, K

V :

Velocity, m/s; voltage, V

$\dot{W}$ :

Shaft work rate, kW

α :

Mass fraction

η :

Energy efficiency

λ :

Tip-speed-ratio

ρ :

Density, kg/m³

ψ :

Exergy efficiency

ω :

Rotational speed, rad/s

a:

Air

C:

Compressor

CC:

Combustion chamber

C,C:

Cooling cycle compressor

C,H:

Heat pump compressor

Cond:

Condenser

d:

Destruction

des:

Desalination

el:

Electric

em:

Electromechanical

ev:

Evaporator

ex:

Exhaust

f:

Fuel

FW:

Fresh water

FWS:

Fresh water storage

GT:

Micro gas turbine

H2:

Hydrogen

HW:

Hot water

i{i = 1,2,3,…,n}:

State points (or inlet)

opt:

Optimal

OT:

Ocean tidal turbine

st:

Steam

SW:

Salt water

sw:

Fraction salt water

t:

Tidal

WT:

Wind turbine

W:

Wind

w:

Water

a:

Air