Naokatsu Miura

PERFORMANCE OF A HEAT PUMP USING DIRECT EXPANSION SOLAR COLLECTORS

Theoretical and experimental studies were made on the thermal performance of a heat pump that used a bare flat-plate collector as the evaporator. The analysis used empirical equations to express the electric power consumption of the compressor and coefficient of performance (COP), as functions of temperature of evaporation at the evaporator and that of the heat transfer medium (water) at the inlet of the condenser. The experimental heat pump had a compressor with a rated capacity of 350 W and collectors with the total area of 3.24 m2. Around noon in winter the evaporator temperature was found to be about 17°C higher than the ambient air temperature of 8°C, and a COP of about 5.3 was obtained when the water temperature at the condenser inlet was 40°C. These measured evaporation temperatures and COPs were in good agreement with those predicted by the analysis. According to the analysis, the total area of the collectors in the experiment was appropriate for the heat pump system. Also, the 1-mm thickness of the collectors copper plate used in the experiment could be 0.5 mm with little reduction of COP. The pitch of the tube soldered to the copper plate for the refrigerant flow was 100 mm in the experiment, but the COP would only be reduced by about 4% if the pitch were changed to 190 mm.

Studies of Heat Pumps Using Direct Expansion Type Solar Collectors

An aluminum roll-bond panel with a photovoltaic module on the surface was developed recently for use as an evaporator of a heat pump for residential hot water supply and room heating [Proceedings of the JSES/JWEA Joint Conference, Okinawa, pp. 305-308 (in Japanese)]. In the present study, the panel was modified to reduce the large pressure loss of the refrigerant flow at the evaporator Total area of the panels was 1.9 m 2 and the rated capacity of the compressor was 250 W A collector efficiency factor of 0.9 was obtained for the evaporator The effect of the location of the feeler bulb on performance of the heat pump was also examined.

Studies of Radiative Cooling Systems for Storing Thermal Energy

The thermal performance of an uncovered radiator and a radiative cooling system was investigated experimentally and theoretically. The net radiative power of a black painted surface at ambient air temperature was measured by heat flux plates at night in order to use the results for predicting temperatures of the radiator surface and the fluid in the cooling system on the same night. The net radiative power obtained by the measurements was 40--60 W/m{sup 2} on clear nights in the summer and 60--80 W/m{sup 2} in the fall and winter. The average temperature of the energy storage tank on clear nights became 2--5{degrees}C below the ambient temperature. The experimental and analytical results agreed well with each other.

Studies of a heat pump using water and air heat sources in parallel

The mechanism of heat pumps for hot water supply using dual heat sources of the ambient air and water and the operating conditions of selecting either one or both heat sources were discussed. Then, the performance of a heat pump which used water and the ambient air as the heat sources to heat water was examined experimentally. When the temperature of the water heat source was decreased, the heat from the water as well as the heat from the air was used for the heat pump efficiently until its temperature became approximately that of the evaporation temperature of the heat pump using the ambient air alone as the heat source. When the temperature of the water dropped further, only the heat from the air was absorbed by the evaporators like an ordinary heat pump which used only the evaporator of the air heat source at the same ambient air temperature. A theoretical analysis was made on the performance of a heat pump with dual heat sources. The results agreed well with the experimental results.

Flow Control and Unsteady-State Analysis on Thermal Performance of Solar Air Collectors

Promotion of the use of renewable energy, such as solar heat, for space heating and drying crops and wood is desired to prevent global warming. High-temperature collection of heat by air collectors producing as much as 50°C in winter for space heating and about 80°C in summer for exchanging heat to circulating water for hot water supply would be appropriate applications. In this study, first, a flow control system for constant outlet temperature was installed in a hot air supply system to examine the feasibility of the control system. After experiments, it was found that the control system could function satisfactory. Second, an unsteady-state analysis was made to predict the thermal performance of a flat-plate collector under a given condition of variable flow rate. The analytical model became simple by the assumptions that the heat capacity of the air in the collector and heat conduction in the flow direction through the air and the materials of the collector could be neglected. The maximum differences in outlet temperature and collector efficiency for constant flow rate between the analysis and the experiment were 1.8°C and 6% of the collector efficiency, respectively, except in the beginning of experiments. The biggest difference in the collector efficiency was 30% at 3:00 P.M., which occurred at the end of one of the experiments. The analytical results generally agreed well with the experimental results even when the flow rate and solar radiation changed greatly as time went on. Transient effects are important to predict outlet temperature for variable solar radiation intensity, wind speed, and flow rate.

Studies of a Thermosyphon System with a Heat Source Near the Top and Heat Sink at the Bottom

Thermosyphon are heat transport devices that can transfer heat using gravitation. They can work without any external power supply, and have simple structures. Recently, a device which transferred heat from the hot reservoir near the top to the cold reservoir at the bottom was invented by Ippohshi et al.[1] In this study the same type of device was made and the performance was examined. Then, another type of device which was simpler to make was proposed and the performance was compared with each other. It was found that the one proposed in this study did not take time before the water in the tube circulated and that there was a possibility for applying the device to a solar water heating system which would work without a pump.

Performance of Hybrid Photovoltaic and Thermal (PV/T) Collector With and Without Electricity Generation

This paper presents the experimental results that involve the analysis of a photovoltaic and thermal (PV/T) hybrid collector, which has an integration of the photovoltaic cells and a solar thermal panel into one collector, by focusing on its thermal efficiency when both thermal and electrical energies are generated and when only thermal energy is generated. The experiment was performed at 4 different collector input temperatures, i.e., 12°C, 15°C, 20°C, and 25°C under the condition that the water flow rate was 4 l /min. We analyzed during the period of peak irradiation from 12:00 to 13:00 to obtain the semi-steady-state thermal performance. Besides, in order to remove the differences of the wind environment, the wind heat losses are calculated in the analysis. The result shows that the thermal efficiency of the PV/T collector decreases slightly when only thermal energy is generated as compared to that when both thermal and electrical energies are generated.Copyright

A Solar Heat Pump Using Direct Expansion Type Collectors : Use of Flat Plate and Finned Tube Collectors

Global warming due to the greenhouse effect is a major obstacle to achieving a sustainable society. The use of clean energy such as solar energy and wind power energy contribute to reduction in the greenhouse effect. Solar heat can be used to increase the thermal performance of a heat pump that adopts a collector as the evaporator. Morrison reported the performance of a heat pump water heater with solar boosted evaporators, and simulated the performance of a heat pump with a water storage tank into which the condenser was integrated (Morrison, 1994). This system eliminates the parasitic energy of the pump used to transport heat from the heat pump condenser to the water storage tank (Morrison, 1999). Ito et ale reported the performance of a heat pump using an aluminum roll-bond flat-plate collector (radiation solar collector), which was designed for a PV-thermal hybrid, as the evaporator (Ito, 2003). To eliminate the fall in performance of the heat pump at low solar radiation intensity, flat-plate collectors and air-refrigerant heat exchanger were used together (Ito, 2004). However, a fan was required for the air-refrigerant heat exchanger. In this research, a finned tube collector (convection solar collector) was used instead of an air-refrigerant heat exchanger. A finned tube collector does not need a fan, and absorbs solar heat as well as heat from the ambient air. This paper presents a comparison of experimental and analytical results of COP (coefficient of performance) for a heat pump using flat-plate collectors and a finned tube collector arranged in parallel. The degree of superheat at the exit of the evaporator of the heat pump may affect the performance of the heat pump. Therefore, the analysis of the heat pump performance considered the degree of superheat. In the analysis, the influence of solar radiation intensity, wind velocity and ambient air temperature on the COP of the heat pump using the flat-plate collectors, a finned tube collector or combination of these two types of collectors arranged in parallel was investigated.

Flow Control and Unsteady State Analysis on Thermal Performance of Solar Air Collectors

Solar air collectors can be used for heating rooms, drying crops and wood, and heating water. In present studies, first, a flow control system for obtaining a constant temperature at the exit of a flat-plate collector was installed in a hot air supply system and the feasibility of the control system was examined. When the flow temperature was lower than the temperature that was set by a digital indicating controller, the fan power was reduced to decrease the flow rate. When the outlet air temperature was higher than the setting temperature, the flow rate was increased. Consequently, the control system worked well to maintain the exit air temperature. Secondly, an unsteady state analysis was made on thermal performance of the collector. In analysis, flow rate variations over time were given as conditions. Validity of this analysis was checked by experimentation. Analytical results agreed well even when solar radiation intensity, wind speed, or flow rate changed abruptly.Copyright

Approximate solution and experimental investigation of the thermal performance of a latent heat thermal energy storage unit with a finned tube.

An approximate solution is proposed for the heat transfer problems of a latent heat thermal energy storage unit with a finned tube through which a coolant flows. In the actual storage unit, the temperature of the coolant changes along the axis of the tube so that the problems have been very difficult to solve. This analysis uses a dimensionless heat extraction rate or an apparent Biot number as a function of a dimensionless time, which has been obtained by analysis or experiment for the finned tube at a constant wall temperature. For this purpose, and analysis was made on the apparent the performance of a thermal energy storage unit with the finned tube and n-Octadecane as the phase change material agreed well with the experimental results.