Variable-Speed and Multi-Mode Solar Assisted Heat Pumps: System Design and Controls Development
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Date
2024-06-13
Authors
Howarth, Julian Craig Peter
Journal Title
Journal ISSN
Volume Title
Publisher
University of Waterloo
Abstract
In an era characterized by increasing energy cost and frequent reminders of the climate impact of emissions from combustion, consumers and energy regulators have a heightened interest in solar and other renewable energy sources. This thesis details work that was undertaken to investigate the performance of a Solar-Assisted-Heat-Pump system (SAHP) for Domestic Hot Water (DHW) made novel by the incorporation of a variable capacity heat pump constructed using a 3-phase scroll compressor whose speed can be modulated by a variable frequency drive. The overall purpose of the system being investigated is to meet the DHW load demands of a single-family household, while reducing the annual purchased energy and thereby reducing operating cost and emissions associated with the residence’s DHW consumption.
A key characteristic of the system under study is the variable-speed Heat Pump (HP) which is of custom construction for the current research. A modified factorial experiment was designed and completed to characterize and model the HP’s performance under source and load inlet temperatures and flow rates typical of a mains-connected SAHP system. The resulting multivariate polynomial expressions for HP compressor work rate, source-side heat transfer rate, and load-side heat transfer rate were programmed into a custom component “TYPE” model for TRNSYS, a transient system simulation software package suited to thermal systems. This work represents an improvement on the default HP model in TRNSYS which does not offer the required flexibility to modulate the compressor speed of the HP with a continuously variable input. Validation of the HP model was performed with a separate set of data from those used to fit the model.
A multi-mode SAHP system was modeled in TRNSYS to match an Experimental Test Unit (ETU) housed in the Solar Thermal Research Lab (STRL) at the University of Waterloo (UW). Components of the ETU are commercially available Solar DHW tanks and hydronic heating components. In parallel, a whole system TRNSYS model and a physical system representing a multi-mode variable-capacity SAHP were constructed with model components configured to match their analogous physical components. The TRNSYS model was validated at the component level for the heat pump, Heat Exchanger (HX), Auxiliary Electric Resistance Heaters (AUX), and storage tank. The model was then validated as a whole-system operating over day-long trials under a simplified control regime. Daily validation trials showed agreement between simulated and experimental results.
Annual performance of a variety of configurations, under a temperature-based control scheme consistent with other studies in the literature were studied. The results of these annual simulations showed some performance benefit of the system under study, but highlighted the need for a more advanced control strategy that would make better use of the variable-capacity HP, and correctly decide when the HP should be used over the HX. Poorer performance of the SAHP system than expected was consistent with other studies findings in the literature that also called for more advanced controls.
A Predictive Controller for the variable-SAHP was developed using MATLAB and TRNSYS. The controller works through iterative calls to the TRNSYS system model, the results of which feed into a time-series of control signals that the controller stores and feeds back to the system being operated. Annual simulations were conducted using a top-level TRNSYS simulation in place of the system being operated, and through a MATLAB-link, a separate instance of TRNSYS was used for the iterative sub-simulations. These simulations showed a marked improvement in the performance of the system under the new predictive control scheme compared with simulations of temperature-based control. This improved performance is taken to represent an approximation of the maximum performance of the SAHP system being studied because the predictive controller has selected the optimum control series for the system under perfect simulation conditions. It is acknowledged that in order to maintain realistic performance predictions from the annual TRNSYS simulation, future work is needed to address how the controller would handle model prediction error when controlling a real SAHP system.
As a final verification and demonstration of the work, the new controller created to control TRNSYS simulations was ported to an instance of LABView running on the ETU. The controller was implemented to operate the equipment in the lab as a form of Hardware-In-The-Loop (HIL) simulation. This exercise demonstrates that the concept of predictive control as implemented in this work is capable of controlling a real system under study with the goal of meeting DHW demands. Some disagreement was noted between simulation and experimental operation of the system and explained within the context of limitations of the ETU to reproduce certain losses, and model timing errors that can lead to missed milestones for collection on some poor solar days. A number of suggestions are offered to address the shortcomings uncovered by these verification trials. These suggestions included model improvements, and changes to the controller itself that would make it more robust and capable of dealing with variation between model inputs and the observed conditions.
Description
Keywords
solar-thermal, solar-assisted-heat-pump, heat pump, variable heat pump, TRNSYS, predictive control, energy simulation, domestic hot water, building energy, solar energy, renewable energy