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Comparison of Transcritical CO2 and Conventional Refrigerant Heat Pump Water Heaters for Domestic Applications

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Comparison of Transcritical CO2 and Conventional Refrigerant Heat Pump Water Heaters for Domestic Applications energies Article Comparison of Transcritical CO2 and Conventional Refrigerant Heat Pump Water Heaters for Domestic Applications Ignacio López Paniagua , Ángel Jiménez Álvaro * , Javier Rodríguez Martín , Celina González Fernández and Rafael Nieto Carlier E.T.S. Ingenieros Industriales, Universidad Politécnica de Madrid, c/ José Gutierrez Abascal, 2, 28006 Madrid, Spain; [email protected] (I.L.P.); [email protected] (J.R.M.); [email protected] (C.G.F.); [email protected] (R.N.C.) * Correspondence: [email protected]; Tel.: +34-9106-77189 Received: 21 December 2018; Accepted: 28 January 2019; Published: 1 February 2019 Abstract: Although CO2 as refrigerant is well known for having the lowest global warming potential (GWP), and commercial domestic heat pump water heater systems exist, its long expected wide spread use has not fully unfolded. Indeed, CO2 poses some technological difficulties with respect to conventional refrigerants, but currently, these difficulties have been largely overcome. Numerous studies show that CO2 heat pump water heaters can improve the coefficient of performance (COP) of conventional ones in the given conditions. In this study, the performances of transcritical CO2 and R410A heat pump water heaters were compared for an integrated nearly zero-energy building (NZEB) application. The thermodynamic cycle of two commercial systems were modelled integrating experimental data, and these models were then used to analyse both heat pumps receiving and producing hot water at equal temperatures, operating at the same ambient temperature. Within the range of operation of the system, it is unclear which would achieve the better COP, as it depends critically on the conditions of operation, which in turn depend on the ambient conditions and especially on the actual use of the water. Technology changes on each side of the line of equal performance conditions of operation (EPOC), a useful design tool developed in the study. The transcritical CO2 is more sensitive to operating conditions, and thus offers greater flexibility to the designer, as it allows improving performance by optimising the global system design. Keywords: heat pump water heater; transcritical CO2 heat pump; coefficient of performance; near zero-energy building; modelling 1. Introduction Although CO2 as refrigerant is well known for having the lowest global warming potential (GWP) [1], and fully commercial domestic heat pump water heater systems (HPWH) exist, its long expected widespread use has not fully unfolded [2]. Indeed, CO2 poses some technological difficulties with respect to conventional refrigerants. The high pressure ratio between the evaporator and the cooler necessarily requires double stage compression for reasonable performance, for instance. However, proven technology exists for all devices [3], and other solutions could be studied, for example using CO2 blends, as is being done for power cycles [4]. Moreover, numerous studies show that the coefficient of performance (COP) of CO2 HPWHs can compare favourably to conventional ones [5]. This, however, requires taking into consideration a number of issues in the design and operating conditions of the system. Energies 2019, 12, 479; doi:10.3390/en12030479 www.mdpi.com/journal/energies Energies 2019, 12, 479 2 of 17 Control of the electronic expansion valve EEV as a function fo the refrigerant charge affects COP significantly [6]. Real time control of the discharge pressure as a function of the compressor power consumption, discharge pressure and water outlet temperature can optimise COP [7]. The most sensitive factor for performance and at the same time the greatest potential of CO2 HPWH lies in the heat exchange with the water [1]. This must take place in the 9–10 MPa range, because of the particular location of the critical point in the pressure-enthalpy diagram [5], making the cycle transcritical, and to ensure good thermal conductivity in the CO2 side [3]. The temperature of CO2 in the gas cooler will fall steeply as a consequence, and the temperature profile on the water side will thus determine the efficiency of the heat exchange and of the heat pump. A matching rise of the water temperature will allow high COP, while low temperature rises will cause poor performance. In fact, forcing the heat pump to always heat the water side as much as possible, to cause the greatest stratification in the tank, and regulating the water supply temperature by mixing, can increase COP significantly [8]. An integrated energy system (see Figure1) combining a heat pump, a ventilated façade and a water tank is being designed for a nearly zero-energy building (NZEB) [9]. The exterior side of the ventilated façade is made of photovoltaic (PV) panels which will generate part of the total energy consumption. The panels will operate at a higher efficiency because they will be refrigerated by the ascending air between the façades. The warm air arriving at the top of the building, instead of being let to the ambient, will be collected and circulated through the evaporator of an air to water heat pump, which will produce hot air and water for the building. Due to the warm temperature at the evaporator, the efficiency of the heat pump will increase with respect to a conventional setting in which the evaporator operates with ambient air [10]. In a first setting, the condenser will heat water stored in a tank, but a second alternative with a latent heat thermal storage system (LHTSS) is considered [11]. The operating temperatures at this side will fall in the 55–60 ◦C range [12], usual for domestic HPWH [10]. Figure 1. Diagram of the integrated energy system for buildings. The evaporator (d) receives warm air from the air ascending through the air chamber (b) after refrigerating the panels of the outer façade (a). The hot side of the pump is either a water tank (c.1) or a latent heat thermal storage system (LHTSS) (c.2). The system has been designed for a R410A-based HPWH, although it would be interesting to study a transcritical CO2-based HPWH (TC-HPWH) alternative. The performance of CO2 heat pumps is very sensitive to operating conditions [13], basically due to the temperature profile of the CO2 at the Energies 2019, 12, 479 3 of 17 gas cooler. It is presumed that, for the application considered here, where evaporator temperatures will be warm and water outlet temperatures moderate, the CO2 heat pump could result advantageous over the conventional at some point [14]. However, this is not straightforward to assume beforehand, as the water inlet temperature will also be moderately high, limiting the water temperature rise at the gas cooler, and thus the gas cooler performance. On the other hand, although this could favour the performance of the R401A over the CO2, the water outlet temperature will be on its upper limit, while well within the operating range of the CO2 heat pump. This work will compare the thermodynamic cycles of two commercial, domestic R410A and CO2 HPWH. A pattern of behaviour will be extracted, which is assumed to scale to larger systems. Parting from experimental studies, both cycles have been modelled independently. As their original topologies were different at the high-pressure end, a hypothetical common topology and a range of operating conditions have been defined for the study. 2. Methodology The R410A-based HPWH was a Daikin EKHHP300AA2V3 (Osaka, Japan), and the TC-HPWH was the Sanyo EcoCute SHP-C45DEN (Osaka, Japan). While the conventional HPWH uses a conventional inverse Rankine cycle, the TC-HPWH cycle was modified. Both cycles were modelled with original code using Engineering Equation Solver (EES), assuming the following hypotheses: • Steady-state operation • Negligible pressure losses in piping and heat exchangers • Piping, compressors and expansion valves are adiabatic • Negligible heat losses to the environment in the gas cooler and water tank Specific hypotheses and correlations were assumed for each HPWH, which are discussed in the following sections. 2.1. Conventional R410A Heat Pump Model As mentioned above, the Daikin EKHHP300AA2V3 uses a conventional inverse Rankine cycle to heat a 300-L water tank (see Figure2). The manufacturer offers some performance data of interest for this study [15,16], some of them extracted in Table1. Figure 2. Block diagram of the Daikin EKHHP300AA2V3, R410A-based heat pump water heater systems (HPWH). Measured variables and sources of energy consumption are indicated. Energies 2019, 12, 479 4 of 17 Table 1. Manufacturer’s operation and performance data for the Daikin EKHHP300AA2V3. Parameter Value COP 4.3 Maximal operation pressure 41.7 bar Ambient temperature op. range 2–35 ◦C Two aspects of the heat pump must be modelled: first, the cycle itself, which is described in Section 2.1.2; and, second, the way in which the heat pump adapts to the cold source (environmental temperature) and to the hot source (water tank temperature). The temperature gap between the saturation temperature at the evaporator T3 and the environmental temperature T0 is determined by the way in which the controller operates the cycle, and is therefore given by design. It can only be determined by experimental readings. This is developed in Section 2.1.3. Similarly, how the controller adapts the condensing temperature T1 to the water tank temperature T7 is also given by design, although this aspect was only of marginal importance for this study. 2.1.1. Experimental Setup Several sensors were fixed to the Daikin EKHHP300AA2V3 to obtain the temperatures at all significant points of the cycle (T1 − T4). The evaporator pressure P3 is the saturation pressure at T3, because the compressor receives vapour from a two-phase separator. The global consumption of the heat pump W˙ was measured. Air velocity, air in and out temperatures at the evaporator (T5 and T6, respectively) were also registered. The temperature of the room (T0) was controlled to force the heat pump to operate at the desired range in the evaporator.
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