Improving the performance of booster heat pumps using zeotropic mixtures

Abstract

This study demonstrated an increase in the thermodynamic performance of a booster heat pump, which was achieved by choosing the working fluid among pure and mixed fluids. The booster heat pump was integrated in an ultra-low-temperature district heating network with a forward temperature of 40 °C to produce domestic hot water, by heating part of the forward stream to 60 °C, while cooling the remaining part to the return temperature of 25 °C. The screening of working fluids considered 18 pure working fluids and all possible binary mixtures of these fluids. The most promising solutions were analysed with respect to their performance under off-design conditions and their economic potential. The best-performing mixture showed a coefficient of performance (COP) of 9.0 and thereby outperformed R134a by 47 %. Although the mixed working fluids resulted in higher investment cost, the economic performance was comparable to the pure fluids. The mixtures showed similar performance as the pure fluids at off-design conditions. It was concluded that the mixtures 50 % Propylene / 50 % Butane and 50 % R1234yf / 50 % R1233zd(E) could considerably improve the thermodynamic performance of the overall heat supply system while being economically competitive to pure fluids.

Analysis of possibilities to utilize excess heat of supermarkets as heat source for district heating

“…The solutions using a cascade heat pump show longer payback periods while the standard unit using Ammonia approaches the payback periods of the direct heat recovery. Other performance indicators that account for the entire life time, such as the heat generation cost or the net present value indicate a higher profitability of the cascade heat pump solutions, which is consistent with the findings from [5,22]. The estimation of the investment cost for the establishment of the DH connection implied uncertainties, which have the largest impact on the solution with the direct heat supply from the refrigeration system.…”

“…Estimating the investment cost when using mixtures appears to be more difficult due to the lack of practical experiences with the construction and operation of heat pumps using zeotropic mixtures. Previous studies, [5,22], suggest initial investment costs of competitive solutions using mixtures, comparable to the investment costs for heat pumps using pure fluids or up to 100 % higher. The studies outline the strong dependency on the specific cases.…”

“…The authors have shown in previous studies that utilizing mixtures constitutes a promising approach to match the temperature profile of the working fluid during evaporation and condensation to the temperature profile of heat sink and source, [5,21,22]. This approach enabled reducing the irreversibilities during heat transfer and thereby improving the performance.…”

“…A working fluid screening was conducted as described in [5,21,22]. The model for the cascade heat pump was therefore evaluated for several mixtures, considering all possible binary mixtures of a list of fluids including 14 natural fluids and 4 hydrofluoroolefins (HFOs).…”

“…Several studies have therefore compared various DH technologies under consideration of different conditions, such as consideration of existing network components, availability of heat sources, network layouts and other aspects [2][3][4][5].…”