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Entropy 2021, 23, 47 8 of 17 3. Exergy Efficiency Calculations Exergy efficiency for a steady-state process occurring in a heating and/or cooling system can be calculated based on the following equation [28]: ηex = Exergy in product outputs (1) Exergy in inputs Based on this equation, overall exergy efficiency, considering heating and cooling processes, has been calculated as well as partial exergy efficiencies for space heating and cooling and sanitary hot water production. Overall exergy efficiency of all systems has been calculated on a basis of the following equation: ηex,ov = ∑Eh +∑Eshw +∑Ec (2) ∑Ein where ∑ Eh, ∑ Eshw, ∑ Ec denote sum of exergy in product outputs over a time period, i.e., exergy of heat, required for space heating and sanitary hot water production, and exergy of cooling energy, required for space cooling. The value ∑ Ein denotes exergy in inputs, i.e., electrical energy in the following cases: 1. HPs for heating (space heating of building and SHW production) and cooling purposes; 2. In the case of pumps connecting SHS with ST collectors and GW; 3. Exergy of the energy carrier in the case of natural gas heating systems. All the partial exergy efficiencies have been calculated in the same manner, excluding parts that are not used during the calculations. The quantity of exergy of the heat can be evaluated by the work output of a Carnot heat engine, which operates between the hot reservoir and the ambient temperatures: Th/shw −Ta Eh/shw = T Qh/shw (3) h/shw Exergy of cooling energy has been calculated using a similar equation: Ta −Tc Ec = T Qc (4) c In case of the heating system based on NG, the exergy in inputs, i.e., the exergy of heat, produced by a NG boiler, has been calculated by considering the following fact: according to Rant [29], the ratio of difference of specific exergy and superior calorific value in relation to superior calorific for methane (CH4) at the reference temperature of 25 ◦C is: eCH4 − Hs,CH4 = − 0.0815 (5) Hs,CH4 This ratio shows that the typical exergy efficiency of heat produced by a NG boiler is (1–0.0815), i.e., approximately 92%. 4. Results and Discussion In this section, the results of the performance and exergy analysis for five different cases of heat and cooling energy production with different systems in a four-family house are presented below. 4.1. Performance Analysis of Building Simulation In order to determine the exergy efficiencies of all presented systems, several other parameters had to be determined by using the simulation tool TRNSYS. Systems B, C and D considered heat pump systems for heating and cooling energy production. The heat source for System B (heat pumps A–W, W–W and G–W) were ambient air, ground water and ground (brine). Its annual temperature variation is shown in the Figure 5. In relation to ambient air temperature, the annual heating and cooling demand isPDF Image | Solar Assisted Heat Pump with Seasonal Heat Storage
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