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Energies 2021, 14, 4103 15 of 28 The economic viability of the designs is evaluated by means of the net present value (NPV) and discounted payback period (DPP), defined in Equations (8) and (10), respectively. NPV is a method to represent the discounted cash flow, which is defined as the sum of net cash flows over the plant economic life, N, calculated as i ∑NCe(1+re)t NPV=C + (1+rd)t (8) t=0 where rd is the average annual effective discount rate (cost of money), and re is the general inflation rate of electricity prices. The net cash flow is represented by the initial investment cost, Ci, and the sum of all operational incomes over the system’s lifetime, which in this analysis amount to the saved electricity expenses, Ce. The latter is defined in Equation (9). Ce=p(ESH+EDHW + EAC −ESH+EDHW+EAC) (9) ηel COPAC,al COP An existing reference system (capital cost = 0), consisting of an electric boiler and an alternative standard AC cooling unit, is applied for the analysis. It is assumed that all SH and DHW energy requirements, ESH and EDHW, are covered by the electric boiler, while all cooling energy, EDHW, is covered by the alternative AC chiller. The DPP, defined in Equation (10), determines the time from investment to return of the invested capital. DDP is calculated by identifying the year, Yn, in which the proceeding cumulative net cash flow (NCF), ∑nt=0 NCFn, turns positive. The exact time of return is found by accounting for the discounted value of the cash flow of the next period, NCFn+1. DPP = Yn + abs(∑nt=0 NCFn) (10) NCFn+1 Data applied in the economic analysis are listed below. • The general inflation rate is re = 2.5% [53]. • The average annual effective discount rate is rd = 10% [53]. • The plant economic life is N = 15 years [52,53]. • The electric boiler efficiency is ηel = 95% [56]. • The European seasonal energy efficiency ratio (ESEER) of the alternative R134a AC cooling system is ESEER = COPAC,al = 2.52 (from manufacturer catalog). • Costs related to system maintenance and operation have been neglected [52,53]. 4. Results and Discussion 4.1. System Performance and Operation Transient simulations were conducted for the purposed CO2 systems with the bound- ary conditions and control schemes described in Section 3. Figure 8 compares the COP of the investigated designs as a function of ambient temperature and charging strategy. Naturally, the COP for all designs increases with ambient temperature as the pressure ratio of the compressors diminishes. At 20 ◦C, the COP of all designs increases considerably to values in the range of 7.3 to 7.6 and 5.8 to 6.3 for leveled and aggressive charging, respectively. This is explained by the presence AC cooling demand during high ambient temperatures, which enables combined heat pump and chiller operations. The COP of all designs are gradually reduced from 25 ◦C, due to the increase in AC cooling loads relative to the DHW production load. Figure 8 clearly illustrates that the system with the ejector arrangement, EJ, outper- forms both SC and PC, independent of charging strategy. A more significant benefit is achieved from the ejector at ambient temperatures above 15 ◦C, where the performance is enhanced by up to 20% and 14% compared with SC and PC, respectively. Moreover, leveled charging results typically increased COP compared with aggressive charging due to continuous DHW charging during all operational hours. In contrast, DHW productionPDF Image | Evaluation of Integrated Concepts with CO2 for Heating
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