Evaluation of a Series-Configuration Solar-Assisted Heat Pump with Performance-Predictive Controls
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Date
2017-10-18
Authors
Chasse, Christopher Thomas
Journal Title
Journal ISSN
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Publisher
University of Waterloo
Abstract
This thesis evaluates a configuration of a 3-mode solar-assisted heat pump (SAHP), which features a heat exchanger and heat pump which may operate independently or in series to provide heat for a domestic hot water system. The system is designed to deliver heat through the heat exchanger, heat pump, or both simultaneously, with the operating mode dictated by a system controller. The configuration is evaluated in comparison to established system designs, such as electric-only, solar domestic hot water (SDHW) and 2-mode SAHP (heat exchanger or heat pump) systems. TRNSYS 17 simulation software is used to perform year-long simulations of the systems for performance comparison purposes. Subjecting each system to the same parameters, weather conditions and draw patterns ensures that differences in performance are due to the available modes of heat delivery.
The systems simulated in TRNSYS use mathematical models of physical components to model the behaviour of a real system. Most of the models used were previously validated by former student William Wagar, supporting their use here. The system includes a heat exchanger and heat pump that are connected to operate independently or in series, depending on the settings of several flow diverter valves. The inclusion of the series configuration is intended to maximize heat gains in high-demand scenarios and to extend heat pump operating hours in low-availability scenarios.
A highly-detailed decision-making scheme is developed to control the system. The scheme uses measurements of available sunlight and system temperatures to predict the performance of individual components, using validated equations and specified system settings. The predicted performance values are used, in conjunction with an evaluation of the current system demands for heat, to determine the optimal mode of operation at that time. The “optimal” mode may be the mode which delivers the most heat, consumes the least electricity, or has the best heat delivery to electricity consumption ratio (COP), depending on the demand; the highly effective control strategy balances these priorities to achieve high solar gains and energy savings, while maintaining high temperatures in the storage tank.
The behaviour of multi-mode SAHP systems, when contrasted with single-mode designs, shows that having the option to use a heat exchanger and a heat pump substantially improves solar gains and electricity savings. Such systems have the flexibility to adapt to changing weather conditions to ensure strong performance at all times. The behaviour study shows that under the current control settings, the series configuration is rarely utilized by the system. Additionally, system behaviour is dominated by the maximum heat delivery mode, with rare consideration of power consumption. A parametric study indicates that SAHP systems operate most effectively with a constant thermostat setting of 60 C. A location/climate study indicates that SAHP systems are best-suited to settings in which SDHW systems fare poorly, such as the sub-arctic climate of Whitehorse, Yukon, Canada.
A highly detailed control scheme with the ability to apply several definitions of “optimal performance” is shown to allow excellent system performance in comparison to single-definition designs. The results of this study indicate that the series mode is a beneficial but rarely-used option, though use of this mode may be increased if a smaller heat pump is installed. It is recommended to focus on further development of the control scheme by optimizing system settings. It is also recommended to perform tests of the system, using an appropriately-sized variable-speed heat pump, to validate the controller’s feasibility and further advance the field of SAHP study.
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Keywords
Solar Assisted Heat Pump, Solar Domestic Hot Water