Utilization of the heat pumps for drying is interesting and very useful for commercial and residential purpose. Heat pump dryers possess high coefficient of performance and potential improvements in the quality of dried products. The traditional heat pumps are driven by electricity or heat, which strongly increases the consumption of electricity and fossil energy.
Solar heating system
Solar energy is acknowledged as the most effective form of renewable energy in those applications in which the thermal or electrical power required is compatible with spatial and economic constraints. Solar drying technology offers an alternative, which can process the products in clean, hygienic and sanitary conditions with zero costs, abundance in nature, cheap, renewable, low environmental impact and represents a good source of thermal energy. Nevertheless, the solar drying systems has some following limitations. First, the intensity of solar radiation is a function of time, so thee are overheating and/or under-heating of working fluid. Second, solar radiation has low energy density, which requires use of large energy-collecting surfaces. Thirdly, solar air dryer systems cannot work in off sunshine hours and cold weather.
Therefore, it is necessary to develop an energy efficient drying system to overcome the above limitations.
Combined solar dryer and heat pump can overcome these difficulties and satisfy important demands in industrial drying with respect to product quality control, reduced energy consumption and reduced environmental impact. Reduced energy consumption is achieved due to the high coefficient of performance of the heat pump and the high thermal efficiency of the dryer when properly designed. In the conventional heat pumps, if the evaporator is enclosed in solar heater, large amount of heat will be released in the condenser. Therefore, Solar Assisted hybrid Heat Pump system is required to develop for reducing the energy consumption and enhance the overall coefficient of performance of the system.
Solar assisted hybrid heat pump
In solar air dryer, all traditional devices, storage and collector temperature are directly related to each other, since it is possible to heat the storage (drying chamber) only when the collector temperature is higher than storage one. This constraint limits the thermal efficiency of the collector and at the same time, it reduces the minimum solar radiation usable for heating, which must be high enough to compensate heat losses towards the environment. If the heat is produced at excessively high temperatures, a significant part of the solar energy is lost towards the environment. if the temperature of the panel remains close to that of the environment, the maximum heat flux into the storage tank is obtained; though, this heat will have very low effectiveness (exergy value), since the storage temperature is low. To break the link between collector and storage temperatures and to overcome the solar thermal collector limits, the insertion of a heat pump has been conceived and applied in the so called Solar Assisted Heat Pump (SAHP) device.
Solar assisted heat pump system is composed of Vapour Compression Cycle, which is combined with a solar collector and this combined system possesses a high coefficient of performance. This system consists of a solar collector, a heat exchanger as condenser, expansion valve (capillary type) and a compressor. The solar collector is used as the evaporator which is roll-bone type of the heat pump system. The refrigerant is directly vaporized in the solar collector–evaporator due to the solar energy input, where phase change from liquid to vapour occurs. Due to the heat pump, the collector temperature is kept close to or below the environmental temperature and the temperature of the drying chamber close to the desired temperature. This solution yields; evaporator/collector temperature is kept more uniform, owing to the phase change which the refrigerant fluid undergoes in the roll-bone evaporator, the fraction of captured solar energy increases, due to the ability of the evaporator/collector to operate at low temperatures, also with low solar radiations (even during night-time). Indeed, the collector efficiency increases due to the reduced heat losses towards the environment. This leads to several advantages compared to conventional heat pump and also solar air dryer system.
- The direct vaporization of the refrigerant in the solar collector/evaporator leads to higher heat transfer coefficients.
- Using refrigerants as the working fluid in the heat pump cycle results with low temperature during the evaporation process in the solar collector, which leads to lower system losses since the collector loss value is a function of the collector to ambient temperature difference.
- The collector, including bare flat-plate collectors, works at high efficiency values based on the low collector to ambient temperature differences, which also reduces collector cost.
Based on this principle, attain a high coefficient of performance value with reducing the compressor power consumption and enhancing the global system performance.
Performance investigation of solar hybrid heat pump
- The study shows that when we use evaporator as a solar collector, the output temperature of hot air on condenser side increases. Figure 4 clearly shows that with the Solar hybrid heat pump, the hot air temperature was considerably high compared to hot air temperature with conventional air source heat pump.
- Furthermore, when we integrate evaporator as a solar collector, the significant increase can be shown in Coefficient Of Performance (COP) of Solar Hybrid Heat Pump System when compared to conventional Heat pump System in presence of solar radiation.
Conclusion
Utilization of the heat pumps for drying is interesting and very useful for commercial and residential purpose. Heat pump dryers possess high coefficient of performance and potential improvements in the quality of dried products. The traditional heat pumps are driven by electricity or heat, which strongly increases the consumption of electricity and fossil energy. This study surely delivers the solar assisted hybrid heat pump system, which is able to perform at higher hot air temperature as well as higher COP as compared to conventional heat pump system.
Frank Gandhi has done B.Tech in Mechanical Engineering from Pandit Deendayal Energy University (Formerly PDPU), Gandhinagar. Currently he is working at Avant Garde Cleanrooms & Engg. Solutions Pvt. Ltd. (ACES) having vast experience in HVAC&R industry. He has provided technical solutions related to HVAC, Refrigeration and Cleanroom for various industries like Pharmaceutical, Solar, FMCG, Commercial, Industrial, Hospitality etc. He is also a prominent member of ISHRAE.
