Stephen J. Harrison

The Effect of Hybrid Photovoltaic Thermal Device Operating Conditions on Intrinsic Layer Thickness Optimization of Hydrogenated Amorphous Silicon Solar Cells

Historically, the design of hybrid solar photovoltaic thermal (PVT) systems has focused on cooling crystalline silicon (c-Si)-based photovoltaic (PV) devices to avoid temperature-related losses. This approach neglects the associated performance losses in the thermal system and leads to a decrease in the overall exergy of the system. Consequently, this paper explores the use of hydrogenated amorphous silicon (a-Si:H) as an absorber material for PVT in an effort to maintain higher and more favourable operating temperatures for the thermal system. Amorphous silicon not only has a smaller temperature coefficient than c-Si, but also can display improved PV performance over extended periods of higher temperatures by annealing out defect states from the Staebler-Wronski effect. In order to determine the potential improvements in a-Si:H PV performance associated with increased thicknesses of the i-layers made possible by higher operating temperatures, a-Si:H PV cells were tested under 1 sun illumination (AM1.5) at temperatures of 25 o C (STC), 50 o C (representative PV operating conditions) , and 90 o C (representative PVT operating conditions). PV cells with an i-layer thicknesses of 420, 630 and 840 nm were evaluated at each temperature. Results show that operating a-Si:H-based PV at 90 o C, with thicker i-layers than the cells currently used in commercial production, provided a greater power output compared to the thinner cells operating at either PV or PVT operating temperatures. These results indicate that incorporating a-Si:H as the absorber material in a PVT system can improve the thermal performance, while simultaneously improving the electrical performance of a-Si:H-based PV.

Characterization of a Thermosyphon Heat Exchanger for Solar Domestic Hot Water Systems

This paper presents a simplified test method that was developed to allow preconfigured solar domestic hot water systems that use natural convection/thermosyphon heat exchangers to be characterized. The results of this test method produce performance coefficients for simple empirical expressions that describe the fluid flow and heat transfer in the heat-exchange loop. These empirically derived coefficients can be used as an input to a general simulation routine that allows overall system performance to be determined for various loads and climatic conditions. To illustrate the test procedure, results are presented for a typical heat exchanger under a range of operational conditions.

Experimental Characterization of a Natural Convection Heat Exchanger for Solar Domestic Hot Water Systems

To predict the long-term performance of solar domestic hot water (SDHW) systems requires computational models that can characterize the systems under a range of operating conditions. The development of detailed fundamental models that suitably describe the operation of systems with natural convection heat exchangers is, however, difficult and time consuming. The fact that the natural convection flow through the heat exchanger is intrinsically self-controlling and temperature dependent complicates the analysis. One approach to modeling this type of system is to use performance characteristics, empirically derived from experimental data, to predict the performance of the heat exchanger under typical operating conditions. Unfortunately, a significant number of tests may be required to characterize the full operation of the device. This paper presents a simplified test method that was developed to allow pre-configured SDHW systems that use natural convection heat exchangers, to be characterized. The results of this test method produce performance coefficients for simple empirical expressions that describe the fluid flow and heat transfer in the heat-exchange loop. These empirically derived coefficients are an input to a general simulation routine that allows overall system performance to be determined for various loads and climatic conditions. In this paper, data is presented for a typical heat exchanger under a range of operational conditions.© 2006 ASME

Modeling of an Indirect Solar Assisted Heat Pump System for a High Performance Residential House

Heat pumps are commonly used for residential spaceheating and cooling. The combination of solar thermal and heat pump systems as a single solar-assisted heat pump (SAHP) system can significantly reduce residential energy consumption in Canada. As a part of Team Ontarios efforts to develop a high performance house for the 2013 DOE Solar Decathlon Competition, an integrated mechanical system (IMS) consisting of a SAHP was investigated. The system is designed to provide domestic hot water, space-heating, space-cooling and dehumidification. The system included a cold and a hot thermal storage tank and a heat pump to move energy from the low temperature reservoir, to the hot. The solar thermal collectors supplies heat to the cold storage and operate at a higher efficiency due to the heat pump reducing the temperature of the collector working fluid. The combination of the heat pump and solar thermal collectors allows more heat to be harvested at a lower temperature, and then boosted to a suitable temperature for domestic use via the heat pump. The IMS and the buildings energy loads were modeled using the TRNSYS simulation software. A parametric study was conducted to optimize the control, sizing and configuration of the system. This paper provides an overview of the model and summarizes the results of the study. The simulation results suggested that the investigated system can achieve a free energy ratio of about 0.583 for a high performance house designed for the Ottawa climate. Copyright

Investigation of Reverse Thermosyphoning in an Indirect SDHW System

Thermal energy storages with thermosyphon natural convection heat exchangers have been used in solar water heating systems as a means of increasing tank stratification and eliminating the need for a second circulation pump. However, if the storage system is not carefully designed, under adverse pressure conditions, reverse thermosyphoning can result in increased thermal losses from the storage and reduced thermal performance of the system. To investigate this phenomenon, tests were conducted on a single tank and multi-tank thermal storage under controlled laboratory conditions. Energy storage rates and temperature profiles were experimentally measured during charge periods, and the effects of reverse thermosyphoning were quantified. A further aspect of this study was to empirically derive performance characteristics and to develop numerical models to predict the performance of the heat exchanger during reverse thermosyphon operation, and to quantify the relative magnitude of these effects on the energy stored during typical day-long charge periods. Results of this study show that the magnitude of the reverse flow rate depends on the pressure drop characteristics of the heat exchange loop, the system temperatures and the geometry of the heat exchanger and storage tank. In addition, the results show that in the case of a multi-tank thermal storage, the carry over of energy to the downstream thermal energy storages depend on the effectiveness of the exchangers used in the system. Copyright

Comparison of Multi-Tank and Large Single Tank Thermal Storages for Solar DHW Applications

The performance of a multi-tank water storage was studied by experiment and computer simulation. The unit investigated consisted of three 270 L storage tanks connected in series and was charged through individual side-arm, natural convection heat exchangers. Laboratory tests were conducted on a specially instrumented prototype to characterize its performance in terms of temperature stratification, heat transfer and energy storage rates. Based on these tests, a computer model of a complete multi-tank solar thermal system was created. With this model, the performance of a multi-unit storage was compared to a single-tank system of equal total storage volume for a multi-family solar domestic hot water (SDHW) system application. Data were produced for two U.S. locations representative of differing climatic locations. Results show that a reasonably insulated multi-tank system can be used in place of a large single tank with only a small reduction in delivered solar energy.Copyright

Non-Isothermal, Flat-Plate Liquid-Desiccant Regenerators: A Numerical Study

Solar energy can play a significant role in increasing the environmental sustainability of air-conditioning systems and a number of thermally-driven solar air-conditioning technologies are available. Among these technologies, liquid-desiccant systems have the potential to operate efficiently at lower regeneration temperatures, allowing for better use of flat plate collectors. In these systems, air to be conditioned is dehumidified through direct contact with a desiccant solution. This solution is then heated at the regenerator and water is evaporated into a scavenging air stream. This paper presents an analysis of the heat and mass transfer in a parallel-plate internally-heated liquid-desiccant regenerator. Using a finite-difference technique, a numerical analysis was performed to solve the species and energy equations. The influence of regenerator hot water parameters (temperature and mass flow), and solar collector efficiency on regeneration capacity and specific energy consumption was evaluated.Copyright

Experimental Study on Natural Convection in an Asymmetrically Heated Inclined Channel With Radiation Exchange

Heat transfer in an asymmetrically heated, inclined channel by natural convection and radiation exchange was experimentally investigated. Experiments were conducted on channels with small inclination angle (to horizontal) ranging from 18° to 30° and a wall surface emissivity of 0.29 to 0.95. The channel length/space ratio was between 44 and 220. In each test, a uniform heat flux was applied along the top wall of the channel, while the bottom wall was thermally insulated. Temperature profiles along both the top and bottom walls of the channel were recorded under different heat flux and channel length/space ratios. The dependency of maximum wall temperature and heat transfer on the channel spacing and surface emissivity was explored. As a result of this work, correlations of local and average Nusselt number, with modified channel Rayleigh number, were determined and proposed for channels at inclination angle of around 18° and surface emissivities of around 0.95. The proposed correlation will be valid for modified-Rayleigh number in the range of 10 < Ra” < 5.6 × 104 at asymmetric heat flux boundary conditions.Copyright

Evaluation of Cooling Water Storage for Liquid-Desiccant Air Conditioning System

Liquid-desiccant (LD) dehumidification technology has been used to extract moisture from humid air while attempting to consume less electricity than traditional air-conditioning methods. An evaporative cooling tower (ECT) was used as a cooling device to reject the latent heat gained by the system to regenerate the desiccant solution. The performance of an ECT was evaluated both experimentally and through TRNSYS simulations to investigate optimal operating conditions. The ECT often operated in humid conditions which resulted in reduced heat rejection rates and ineffective operation. To improve performance, cooling water storage (CWS) was investigated as a way to reduce ECT usage during periods of higher ambient humidity. To undertake this study, the complete LD system, incorporating CWS, was modelled in TRNSYS for a range of typical operating conditions. The results indicated that operation of the CWS system reduced the electrical power consumption and increased the electrical coefficient of performance (COPE) of the liquid desiccant air conditioning unit system by up to 16%. The total cooling rate improved by up to 6%. Smaller gains in COPT and solar fraction were also found in the simulation results.Copyright

Passive Heat Exchanger Anti-Fouling for Solar DHW Systems

The interior surfaces of heat exchangers used in domestic hot water systems are particularly prone to fouling or complete blockage due to the accumulation of sediments, scale and mineral deposits. In many locations, mineral salts and other impurities may be present in the potable water supply and fouling may occur if the heat exchanger is not routinely cleaned or flushed of accumulated matter. In small residential installations, however, this is not practical due to the associated costs. In response to this need, a passive back-flushing system was designed that allows heat exchangers to be routinely backflushed many times a day. The action is a normal operation of the system and does not require user intervention, external power or controls to function. During back-flushing mineral deposits are washed out of the heat exchanger and flushed from the system. This paper describes the operation of the device and documents the results of accelerated tests undertaken to verify its operation.