Economic analysis of air-water heat pump technologies with a screening method

University dissertation from Stockholm : KTH Royal Institute of Technology

Abstract: Early on in the process of product development a decision has to be made which technologies to focus on. Under a purely techno-economic viewpoint for a heat pump only these technologies should be considered which maximize heat pump performance for a given cost. Finding these optima is in practice far from trivial as the result is inuenced by variations in operating conditions, interactions between components, model assumptions and uncertain economic data. The overall objective of this thesis is therefore to develop a screening method for the evaluation and comparison of different technologies in regard of cost and efficiency with the aim to identify optimal heat pump designs.In practice the applicability of such a method depends on the required effort.  Simple mathematical models and short computation times are as mandatory as reliable, coherent and sufficiently general results. To this end the screening method combines methods of simulation, annual performance calculation, metamodeling and optimization.The screening method is employed for air-water heat pumps in three utilization examples. In the first example the operational costs of maldistribution in evaporators are quantified. It is shown that for air-water heat pumps increasing the evaporator size is no liable option to counteract maldistribution effects. Two alternative technologies, an adjustment of the cycle layout and a change of the superheat control method, are evaluated under the aspect of total cost of ownership. Only with the second technology noteworthy savings can be achieved compared to the baseline. In the second utilization example on/o_ and variable speed control for regulation of the heat pump capacity are considered. Only for colder climates variable speed control pays o_ for the end consumer in a reasonable time. The study shows the comparison of the two control methods to strongly depend on compressor size. In the third example four different cycle layouts are compared. It is demonstrated that the result of the evaluation depends strongly on practical operating limits and on compressor characteristics. The most promising option is a staged layout with economizer heat exchanger.Two additional studies consider the modeling of operating conditions in more detail. In the first study the simple approach for calculating annual performance used in the screening method is compared with a comprehensive dynamic model. Optimization for both model approaches results in similar component sizes. In the second study the evaporator model is extended to include a dynamic frost growth model which is used to assess operational costs induced by frosting and defrosting of the evaporator coil. A method to reduce the number of defrosts without negatively affecting heat pump capacity is presented and its feasibility demonstrated with experiments.The screening method can be extended to include more optimization parameters than used in the presented examples. It can also be applied for other small scale vapor compression systems or it can be integrated in a comprehensive evolutionary optimization procedure to reduce computation time and increase numeric stability. Thus the screening method can be a valuable tool in the product development process for shortening the times until new technologies which safe energy and thereby reduce greenhouse gases are available on the market.