Heat Pump Drying System Research Articles

A solar assisted heat pump drying system for grain in-store drying

For grain in-store drying, a solar assisted drying process has been developed, which consists of a set including a solar-assisted heat pump, a ventilation system, a grain stirrer, etc. In this way, low power consumption, short cycle time and water content uniformity can be achieved in comparison with the conventional method. A solar-assisted heat pump drying system has been designed and manufactured for a practical granary, and the energy consumption performance of the unit is analyzed. The analysis result shows that the solar fraction of the unit is higher than 20%, the coefficient of performance about system (COPS) is 5.19, and the specific moisture extraction rate (SMER) can reach 3.05 kg/kWh.

A review of heat-pump drying (HPD): Part 2 – Applications and performance assessments

In the second part of this study, heat-pump drying systems were comprehensively reviewed in terms of applications and performance evaluations. In this regard, these systems were classified in terms of type of heat pump systems, type of dryer and product types first. The performance assessments included coefficient of performance (COP), specific energy consumption (SEC) and specific moisture extraction rate (SMER), while types of energetic, exergetic and cost analyses were covered. It may be concluded that the most preferred method used to determine the HPD efficiency is SMER, while in recent years exergetic analysis method has been widely used.

Exergetic performance assessment of a ground-source heat pump drying system

In evaluating the efficiency of heat pump (HP) systems, the most commonly used measure is the energy (or first law) efficiency, which is modified to a coefficient of performance (COP) for HP systems. However, for indicating the possibilities for thermodynamic improvement, energy analysis is inadequate and exergy analysis is needed. This study presents an exergetic assessment of a ground-source (or geothermal) HP (GSHP) drying system. This system was designed, constructed and tested in the Solar Energy Institute of Ege University, Izmir, Turkey. The exergy destructions in each of the components of the overall system are determined for average values of experimentally measured parameters. Exergy efficiencies of the system components are determined to assess their performances and to elucidate potentials for improvement. COP values for the GSHP unit and overall GSHP drying system are found to range between 1.63–2.88 and 1.45–2.65, respectively, while corresponding exergy efficiency values on a product/fuel basis are found to be 21.1 and 15.5% at a dead state temperature of 27°C, respectively. Specific moisture extraction rate (SMER) on the system basis is obtained to be 0.122 kg kW−1 h−1. For drying systems, the so-called specific moisture exergetic rate (SMExR), which is defined as the ratio of the moisture removed in kg to the exergy input in kW h, is also proposed by the authors. The SMExR of the whole GSHP drying system is found to be 5.11 kg kW−1 h−1. Copyright © 2006 John Wiley & Sons, Ltd.

Re-circulating heat pump assisted continuous bed drying and energy analysis

Specialty crops such as ginseng, herbs and echinacea need to be dried at low temperatures (30–35°C) for product quality optimization. A drying system that is both energy efficient and preserves product quality is desired. A re-circulating heat pump continuous bed dryer system was designed, constructed and field-tested for this purpose. The heat pump dryer system was experimentally evaluated using several potential herbal and medicinal crops such as alfalfa, catnip, wormwood, red clover, portulaca, dandelion and ginseng. These crops were dried either in chopped, sliced or whole form, depending on the part of the plant (aerial or root). The specific moisture extraction rates (SMER) obtained for various crops were in the range of 0.06–0.61 kg kWh−1. It took 5 days and 190 kWh of energy to reduce the average moisture content of ginseng roots below 10% (wb), while commercial dryers currently in use would take on an average 14 days and 244 kWh of energy at comparable loading rates. The re-circulating nature of the heat pump dryer made it 22% more energy efficient and resulted in 65% reduced drying time compared to conventional dryers incorporating electric coil heaters. Copyright © 2005 John Wiley & Sons, Ltd.

A modular approach to study the performance of a two-stage heat pump system for drying

The performance of a two-stage evaporator heat pump drying system with chlorodifluoromethane (HCFC22) as the working refrigerant was experimentally studied. The system employed low- and high-pressure internal evaporators for cooling and dehumidification of the drying air. On the condenser side, two subcoolers were used for additional sensible heating. The system further incorporated a passive evaporator–economiser for pre-cooling of the air before the evaporators and pre-heating before the condenser. The system performance was evaluated in terms of the amount of heat recovered at the evaporators, the system COP and SMER. It was demonstrated in this paper that more heat, up to 35% more, could be recovered via the two-stage evaporator system in comparison to a single evaporator system. By varying the pressure level of the high-pressure evaporator, greater amount of heat recovered could be realised when the ratio of latent to total load at the two-stage evaporator was improved.

HEAT PUMP DRYING: RECENT DEVELOPMENTS AND FUTURE TRENDS

ABSTRACT This review paper focuses on the recent progress made in heat pump drying (HPD). An introduction is given on the operating and working principle of the heat pump drying system. Advantages and limitations of HPD are outlined. Two industrial applications of HPD, food and timber, are discussed. New developments in HPD are presented. A section discussing the potential of incorporating advanced heat pump cycles for drying application is also presented. Some possible new hybrid HPD technologies, e.g., radio frequency and infrared assisted HPD are discussed in terms of their industrial potential. Opportunities for further R&D to achieve better product quality and energy efficiency are identified. A new heat pump dryer design is also described to demonstrate the new trend in versatile heat pump design.

Performance study of a heat pump dryer system for specialty crops?Part 2: Model verification

The experimental and predicted performance data of a heat pump dryer system is reported. Chopped alfalfa was dried in a cabinet dryer in batches and also by emulating continuous bed drying using two heat pumps operating in parallel. Results showed that alfalfa was dried from an initial moisture content of 70% (wb) to a final moisture content of 10% (wb). The batch drying took about 4.5 h while continuous bed drying took 4 h to dry the same amount of material. The average air velocity inside the dryer was 0.36 m s−1. Low temperatures (30–45°C) for safe drying of specialty crops were achieved experimentally. The heat pump drying system used in this study was about 50% more efficient in recovering the latent heat from the dryer exhaust compared to the conventional dryers. Specific moisture extraction rate (SMER) was maximum when relative humidity stayed above 40%. The dryer was shown to be capable of SMER of between 0.5 and 1.02 kg kW−1 h−1. It was concluded that continuous bed drying is potentially a better option than batch drying because high process air humidity ratios at the entrance of the evaporator and constant moisture extraction rate and specific moisture extraction rate values can be maintained. An uncertainty analysis confirmed the accuracy of the model. Copyright © 2002 John Wiley & Sons, Ltd.

On the steady-state modelling of a two-stage evaporator system

We develop and validate against experimental measurements a steady-state two-stage flooded refrigerant evaporator model for a heat pump drying system. A prototype two-stage heat pump dryer test facility was designed, built and instrumented to provide the required measurements for the validation of the model. Repeatability and data quality tests were conducted to evaluate the accuracy of measurements. Experimental data could be reproduced to within ±6.5 per cent of replicated air and refrigerant side measurements for the same evaporator's air inlet conditions while the discrepancy of energy balance at the air-side and refrigerant-side was observed to be within ±8.9 per cent. The two-stage evaporator model predicted the air-side total heat and latent heat transfer of the two-stage evaporator to within (−6.3 per cent, 7.6 per cent) and (−11.5 per cent, 9.5 per cent), respectively. On the refrigerant-side, the model enabled the calculation of the degree of superheat to within (−10.6 per cent, 1.7 per cent). The model has shown that there is significant improvement in the heat recovered from a two-stage evaporator system compared to a single evaporator system. In addition, the model demonstrated that the improvement in total heat recovery could be as high as 40 per cent over its base-value when the latent to total load at the two-stage evaporator is increased. Copyright © 2001 John Wiley & Sons, Ltd.