Design, Analysis, and Optimization of a Dual Tank Solar-Assisted Heat Pump System

Abstract

This work investigates the performance of a dual tank solar-assisted heat pump (SAHP) system. These systems combine solar thermal collectors (STCs) and a heat pump (HP) into a hybrid system to meet building thermal loads. The main goals of a system of this type are to reduce energy consumption and provide a method to deliver thermal energy with sustainable sources. This research project undertook the design, testing, model creation and tuning, performance evaluation, and optimization of a dual tank SAHP system for domestic hot water (DHW). The novel SAHP configuration developed uses a second thermal storage tank to create more modes of operation with the intent of decreasing the amount of purchased energy.

A key aspect of the work was a test apparatus built at the University of Waterloo (UW) to facilitate equipment testing and mapping, identify operational problems such as HP short-cycling, and tune mode components.

Alternatives were modelled in TRNSYS to develop design recommendations and assess economic justification for the dual tank SAHP system. Despite noticeable electricity savings by these systems, an analysis of tolerable capital cost versus electricity cost shows that SAHP systems are not justifiable for single family dwellings at the current time.

The impact of electricity time-of-use charges was investigated, which revealed that the DHW, SDHW, and single tank SAHP systems consume a similar amount of electricity during off-peak hours. The performance of the dual tank SAHP system in many global climate types was assessed to identify locations best suited for such a system. Many recommendations are provided to improve economic justification of the system.

Overall, the performance benefits of a dual tank SAHP system over the alternatives was demonstrated. Due to the high capital costs of the additional equipment required, economic justification is difficult for a single-family residential application. There are promising applications and system augmentations that should be investigated which can lead to a successful demonstration project of a dual tank solar-assisted heat pump system.