Optimal performance of compression-resorption heat pump systems

A short payback time is required by the chemical process industry for any investments in energy efficient systems, such as heat pumps. Consequently, the operating conditions for implementing compression-resorption heat pumps should be chosen such that the energetic performance is maximal at minimum investment costs. Basically, this should result in a small compressor power, which has a double benefit: low investment in the compressor (one of the main components) and reduced operating costs - therefore also shorter payback times. In this study, the performance of these heat pumps has been numerically investigated for 50 specific industrial cases, by using an equilibrium model for a compression-resorption heat pump (CRHP) with ammonia-water as working fluid. The average concentration of ammonia was varied, while the temperature driving forces between process and heat pump, at the heat exchanger inlets and outlets was kept constant. Temperature lifts up to 124 K can be served with both economic and energetic advantages. The results for the efficiency of the heat pumps obtained from this numerical investigation are compared against a previous model that uses the Carnot efficiency as starting point to estimate the performance under similar assumptions. The results of the simulation show that optimal performance is obtained when the ammonia concentration is chosen such that vapor quality is exactly 100% at the inlet of the resorber. This leads to the lowest compressor power, and consequently to reduced investment and operating costs - resulting in shorter payback times as well as more efficient use of resources, and ultimately to increased industrial acceptance of compression-resorption heat pumps.