Modelling of Flexible Energy Optimized Split Condenser Ammonia Heat Pump

This thesis concern vapor compression heat pumps with plate heat exchangers and split condenser, which is able to produce two sets of temperatures on the sink side of the condenser simultaneously. The split condenser heat pump is compared to a traditional vapor compression heat pump with plate heat exchangers, which is able to produce one single temperature on the sink side of the condenser. The modelling include mass- and energy balances together with correlations for the heat transfer coefficients and pressure drop through the heat exchangers. The compressor is modelled with constant isentropic efficiency, and the expansion valve is modelled as an isenthalpic process. A detailed
numerical model of the condenser unit has been developed as the condenser unit is the only difference between a split condenser heat pump and a traditional heat pump.

The numerical model is validated from experiments performed on a split condenser heat pump at Danish Technological Institute in Aarhus, Denmark. The experiments fit the results from the numerical model with very little error. In this thesis the split condenser heat pump is compared to a traditional heat pump in order to investigate if a split condenser have any advantages with respect to energy consumption or heat exchanger size. The comparison of the two types of heat pumps is based on a number of parameters; coefficient of performance (COP), total heat exchanger area of the condenser unit, total pressure drop on the sink side of the condenser and exergy destruction in the condenser unit. The analysis can be divided into three parts described below:

1. The dimensions of plates in the heat exchangers in the condensing unit of a traditional heat pump are optimized to minimize the total heat exchanger area. This optimized traditional heat pump configuration is used as benchmark through the entire analysis for all configurations of a split condenser heat pump.

2. The same dimensions of plates, as found in the traditional heat pump in step 1, are used in a split condenser heat pump. Boundary conditions such as sink side temperatures and COP are kept the same as in step 1 (the mix of the two outlet sinks in the split condenser heat pump corresponds to the outlet sink of the traditional heat pump). The total area of the heat exchangers in the condensing unit is investigated together with the pressure drop on the sink side for different split conditions (different mass flow rates through the two heat exchangers and different quality of the refrigerant between the two heat exchangers in the condensing unit).

3. The dimensions of plates are set as free parameters in the split condenser heat pump. Temperatures on the sink side and the COP are kept the same as the traditional heat pump in step 1. Pressure drop on the sink side of the split condenser heat pump is set to 50 kPa in both heat exchangers, and the plate dimensions are optimized to minimize the total heat exchanger area in the condensing unit of the split condenser heat pump. This investigation is made for different sink in- and outlet temperatures.

The analysis show that a split condenser heat pump is not beneficial if the plate dimensions are optimized for a traditional heat pump (step 2). The total heat exchanger area in the condensing unit has to be twice as big if the same COP is to be reached. However, the pressure drop on the sink side will become significantly lower in the split condenser heat pump.

If the heat plate dimensions in the heat exchangers are optimized for every split condition in a split condenser heat pump (step 3), the same COP and pressure drop on the sink side of a split condenser heat pump can be reached with a reduced heat exchanger area. The analysis show that the heat exchanger area in a split condenser heat pump can be reduced with 14-24% compared to a traditional heat pump, depending on the temperatures on the sink side of the condenser unit. This analysis only takes the pressure drop in the heat exchangers (and not from pipes and valves) into account.

The investigation mentioned above is a theoretical investigation assuming an infinite number of plate dimensions are available in the production. In a real-life production facility only a limited number of plate dimensions are available. Depending on which plate dimensions are available in the production the potential of reducing the heat exchanger area in a split condenser heat pump is 14-50% compared to a traditional heat pump, assuming that the marginal pressure drop in pipes and valves can be neglected.

A smaller heat exchanger area reduces the total cost of investment on the heat pump. The purchased equipment cost of heat exchangers in the condensing unit is around 10-20% of the total cost of investment of a heat pump. Hence a reduction of for example 20% in heat exchanger area will reduce the total cost of investment by 2-4% assuming that the extra cost of a more complex sink side system with more valves and pipes is not affecting the total cost of investment.

Cases show that large scale SCHP might have the potential of reducing the total cost of investment (TCI) by 15% if the correct plate dimensions are available in the production, but the annual cost is more or less the same. For small domestic heat pump setups, the SCHP have the potential to reduce the total heat exchanger area in by up to 26%. However, the TCI is only 3% lower and the annual cost is the same as for the THP setup. The error from assumptions is higher than the actual savings, hence it is not possible to draw any firm conclusions on what heat pump setup is the most economical feasible.