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Energies 2020, 13, 6576 2 of 24 cannot be reduced by acting over the energy demand without risking the user comfort. Therefore, the only way of reducing the water heating demand consists of using highly efficient technologies, decreasing the energy losses of the system, performing a proper sizing of the integrated components, and applying waste energy recovery strategies to minimize energy losses. There are many technical solutions in order to recover waste heat but heat pump (HP) technology is the only one that could act as a “heat transformer”, getting heat at a given temperature level and supplying heat at the desired temperature level (of course, with the corresponding energy supply). This characteristic will ensure that HPs will play a central role in heating, cooling, and Domestic Hot Water (DHW) efficient applications, as it allows the integration with other heat sources (waste heat, district heating, and the like). In addition, it should be remarked also that it is recognized as a renewable energy resource by the EU [4]. There have been many studies dedicated to increasing the performance of HP technology for DHW, combined with the use of natural refrigerants, especially in the last decade. CO2 (R744) has been the most direct solution in heat pumps for DHW applications. Ref. [5] summarizes the different applications of this approach in the residential sector. Nevertheless, other approaches have appeared based in other natural refrigerants, basically hydrocarbons, in the last years. In [6–8], the optimization of a heat pump performance using propane as a refrigerant is analyzed, controlling the subcooling of the system, and others as [9] in which the substitution of R134a in a HP water heater by propane and isobutane (R600a) is analyzed and optimized. Regarding the waste heat recovery for domestic applications, most works found in the literature only implement the heat recovery from grey water using a heat exchanger [10–15], for instance, in showers. However, the implementation of these kind of systems lose an important part of the wasted energy potential and complementary systems must be installed in order to fulfill the hot water demand. When a HP is used, like in [16], they address this problem acting over the control of the system but they do not address the adaptation of the HP and other components to the system characteristics, losing part of the potential for improvement. When considering a district heating network as a source, an interesting option that is emerging nowadays, as indicated by [17–19], is the combination of ultra-low district heating networks (ULTDH) with a collective HP booster. This option allows to reduce the temperature of the DH network and thus reducing the energy losses as well as increasing the Coefficient of performance (COP) of the HP booster [20]. Considering the HP for DHW production, it is undeniable the high importance and contribution of the thermal energy storage system (TES), as stated in [17] or [18], since it allows the use of small capacity units to cope with relatively high demand peaks. The tandem HP + TES provides a very high efficiency with zero local CO2 emissions, reduces the number of starts-up of the compressor, gives the opportunity to shift generation and demand and hence using low price electricity, and facilitates the integration of renewable energy sources and the recovery of waste heat. When introducing a TES in the system, there are different possibilities, the most common one considered in the literature is the stratified storage tank as stated in [20–28]. The thermal stratification within the water storage tank increases the global energy efficiency of the system compared with other alternatives like the fully mixed storage option [23,24,27] and a bad stratification within the TES system results in a global efficiency drop, as stated in [25,26]. Considering the exergy analysis in a TES system, the results indicate that the stratified tank is a more convenient option [29] than a fully mixed tank. According to [30], the stratification increases the TES efficiency and the exergy storage capacity of the TES system; 2.3 times more usable volume is recovered from a perfectly stratified storage tank as stated in [29]. This has made it that it is the most extended option nowadays. However, it should be taken into account that perfect stratification is not possible. Heat transfer by conduction, but also some irreversibilities, like the association to the process of the water going into and out of the tank, tend to break the stratification, but usually are not considered in the reported theoretical results and could be quite important, resulting in a efficiency loss from the expected theoretical values. Other alternatives, like using a constant temperature variable-water-volume tank, could avoid some of these disadvantages of thePDF Image | Thermal Energy Storage Strategy Booster Heat Pump Low Temp
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