Heat pump dryer Part 3: experimental verification of the simulation

Heat pump dryer (HPD) performance described by computer simulation results in Parts 1 and 2 of this series of papers is here verified by experiments. An HPD designed to be operated in four configurations—two open systems and two partially closed systems—has been built. The variables in the experiments are the air flow rate, the dryer load, the ambient condition (day and night) and, for the partially closed systems, the recirculation air ratio and the evaporator bypass air ratio. The parameters used for the verification are the compressor power, the properties of air entering the dryer, the coefficient of performance, the moisture extraction rate and the specific moisture extraction rate. It is found that the experimental results agree with the simulation results within an acceptable error tolerance. In order to predict the HPD performance accurately, a fine tuning of the compressor model and the pressure drop in two-phase flow of the refrigerant are recommended. © 1997 by John Wiley & sons, Ltd.     

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.     

Experimental verification of a heat pump assisted continuous dryer simulation model

The experimental and predicted performance of a prototype heat pump assisted continuous dryer is reported. The dryer was shown to be capable of specific moisture extraction rates (SMERs) of between 1.5 and 2.5 kg/kWh using wetted foam rubber as the test material being dried. The results highlight the importance of maintaining conditions of high relative humidity within the air stream entering the evaporator; an increase in the relative humidity from 30 to 80% was shown to give a two-fold increase in the SMER. An optimum evaporator bypass air ratio of between 60 and 70% was observed for this dryer. The effects on performance of deviations from this optimum condition were found to be less significant than had been indicated by earlier models. The predicted performance of the dryer using a simulation model developed previously by the authors was in good agreement with the corresponding measured values.     

Heat pump dryer Part 1: Simulation of the models

Heat pump dryer is a complex system because of the interaction of heat and mass transfer of the working fluids. Since the system cannot be completely close, ambient conditions (temperature and humidity) influence the performance of the system. To investigate the performance of the heat pump dryer thoroughly, simulation models of heat pump dryer components have been developed. The finite-difference method was employed in the simulation to examine the state of the working fluids and heat and mass transfer. The simulation of each component can be used to construct different system configurations the results of which are reported in Part 2.     

Calorimeter measurements of a heat pump dehumidifier: Influence of evaporator air flow

The performance characteristics of a Rankine cycle heat pump dehumidifier have been measured in a psychrometric calorimeter which provides close control of the air state. The quality of the measurements was monitored by applying energy and mass balance test criteria. The results show that the influence of the evaporator air flowrate on the moisture extraction rate, and on the energy efficiency of the drier, depends on the relative humidity of the evaporator inlet air stream. When the relative humidity is high the dehumidification efficiency is maximized when the evaporator air flow is maximized, within the range of air flowrates investigated. Below 50% relative humidity the specific moisture extraction rate exhibits an increasingly peaked maximum as a function of the evaporator air flowrate. This maximum moves to lower air flows when the relative humidity is lower.     

Testing of a heat-pump-assisted mechanical opener dryer

In this study a heat-pump-assisted dryer with an opener mechanism was constructed and tested at the Energy Laboratory, Mechanical Engineering Department, Balikesir University, Turkey. The test material used was wet wool, which is a fibrous material. The variables in the experiments were the air rate, the recirculation air ratio (RAR) and the bypass air ratio. In the experiments, the air mass velocities ranged from 0.78 to 1.50 kg/m² s. The bypass air and recirculation air ratios varied from 20% to 80%. For this system both coefficient of performance of the whole system (COP_ws) and the specific moisture extraction rate of the whole system (SMER_ws) increased as evaporator air ratios increased. COP_ws was found to range from 2 to 3.5 while SMER_ws varied between 1.5 and 2.8 kg/kW h depending on RAR.     

An experimental investigation of dehumidifying and reheating performances of a dual‐evaporator heat pump system in electrified vehicles

Climate control system in electrified vehicle is quite different from traditional internal combustion engine vehicle, as it cannot use the wasted coolant heat from engine to keep warm of the cabin. For the dehumidifying and reheating air in the electrified vehicle cabin, the use of a dual‐evaporator heat pump system could improve the total energy utilization efficiency of the climate control system. In this study, an experimental dual‐evaporator heat pump system was set up to investigate dehumidifying and reheating efficiency of a cabin in electrified vehicle. Two dehumidifying modes were chosen in this experiment. One was single‐evaporator mode with indoor evaporator only, and the other was dual‐evaporator mode with both indoor and outdoor evaporators. The dual‐evaporator dehumidifying mode shows higher heat capacity, coefficient of performance, and outlet air temperature, while it has quite lower moisture extraction rate and specific moisture extraction rate compared to the single‐evaporator dehumidifying mode. Therefore, a hybrid dehumidifying and reheating method was suggested as a possible option for realization of energy conservation without sacrifice of thermal comfort for passengers.     

Performance assessment of contact heat pump drying

A comparative analysis is presented of a novel plate‐type isothermal heat pump dryer and a conventional adiabatic heat pump dryer. The energy performance and drying capacity of each dryer type is investigated subject to operational constraints on the maximum acceptable relative humidity and temperature of air passing over the product. The analysis demonstrates that for applicable products, a significant opportunity exists for improving the energy efficiency of heat pump drying, by a factor of 2–3 times compared with current adiabatic heat pump dryers. The moisture extraction rate is also increased in the contact heat pump dryer, by a similar factor. However, these improvements are shown to be sensitive both to the impact of product thickness on heat transfer and to the relative humidity constraint. Copyright © 2010 John Wiley & Sons, Ltd.     

Theoretical performance analysis of heat pump water heaters using carbon dioxide as refrigerant

The aim of this paper is to simulate the performance of an air source heat pump water heater using carbon dioxide (CO₂) as a working fluid. The heat pump water heating system consists of a compressor, a gas cooler, an expansion device and an evaporator. The computer simulation model has been developed by using the heat transfer data and the thermodynamic properties of CO₂. The effects on the heat pump performance by the operating parameters such as the compressor rotational speed, the inlet water temperature at the gas cooler, the inlet air temperature at the evaporator and the mass flow rate ratio of water to refrigerant were presented. For rated capacities of a 4 kW compressor with a 10 kW gas cooler and a 6 kW evaporator, the coefficient of performance is found to be between 2.0 and 3.0. The mass flow rate ratio of water and CO₂ between 1.2 and 2.2 is the most suitable value for generating hot water temperature above 60°C at 15–25°C ambient air temperature. Copyright © 2007 John Wiley & Sons, Ltd.     

太阳能和空气源复合热泵的最优模式切换分析

太阳能和空气源复合热泵在不同模式下具有不同性能,如何在实际应用中以最佳的工作模式运行,对提高系统综合性能至关重要,为此提出2种用于实现系统最优模式切换的判断方法：1）基于最优模式切换的太阳能辐射强度和环境温度的复合环境参数范围表;2）流经2个蒸发器（太阳能集热/蒸发器和风冷蒸发器）的制冷剂质量流量分配比。基于此,在工程方程求解器（EES）中建立太阳能/空气双源辅助热泵热水器的仿真模型。首先,基于上海松江地区的气候条件模拟比较不同模式下的系统性能,再分析和讨论将复合环境参数范围表和质量流量分配比作为系统最优模式切换的判断方法的可行性。结果表明,在相同环境温度下,随着太阳辐射强度的增加,系统最优运行模式将由太阳能-空气模式转变为太阳能模式。此外,将流经2个蒸发器的制冷剂质量流量分配比作为模式切换的准则是可行的,对于本系统其最优运行模式由太阳能-空气模式转变为太阳能模式时的质量流量分配比临界值约为2.02。     

Energy analysis of hazelnut drying system‐assisted heat pump

In this experimental study, a proportional integral derivative (PID) controlled heat pump dryer was designed and manufactured. Heat pump dryer was tested drying of hazelnut and energy analyses were made. Drying air temperatures were changed as 50,45 and 40°C in the drying system. Drying air velocities were changed as 0.25 m s^?1 for 50°C, 0.32 m s^?1 for 45°C and 0.38 m s^?1 for 40°C. Heating coefficient of performance of whole system (COP_ws) of the heat pump dryer was calculated as 1.70 for 50°C, 1.58 for 45°C and 1.40 for 40°C drying air temperatures. Energy utilization ratio changed between 24 and 65% for 50°C, 17 and 63% for 45°C and 14 and 43% for 40°C drying air temperatures in the heat pump dryer. Copyright © 2008 John Wiley & Sons, Ltd.     

Experimental study of gas engine driven air to water heat pump in cooling mode

Nowadays a sustainable development for more efficient use of energy and protection of the environment is of increasing importance. Gas engine heat pumps represent one of the most practicable solutions which offer high energy efficiency and environmentally friendly for heating and cooling applications. In this paper, the performance characteristics of gas engine driven heat pump used in water cooling were investigated experimentally without engine heat recovery. The effects of several important factors (evaporator water inlet temperature, evaporator water volume flow rate, ambient air temperature, and engine speed) on the performance of gas engine driven heat pump were studied in a wide range of operating conditions. The results showed that primary energy ratio of the system increased by 22.5% as evaporator water inlet temperature increased from 13 °C to 24 °C. On the other hand, varying of engine speed from 1300 rpm to 1750 rpm led to decrease in system primary energy ratio by 13%. Maximum primary energy ratio has been estimated with a value of two over a wide range of operating conditions.     

Performance of a gas engine heat pump (GEHP) using R410A for heating and cooling applications

A gas engine heat pump (GEHP) represents one of the most practicable systems which improve the overall energy utilization efficiency and reduce the operating cost for heating and cooling applications. The present work aimed at evaluating the performance of a GEHP for air-conditioning and hot water supply. In order to achieve this objective, a test facility was developed and experiments were performed over a wide range of engine speed (1200 rpm–1750 rpm), ambient air temperature (24.1 °C–34.8 °C), evaporator water flow rate (1.99 m³/h–3.6 m³/h) and evaporator water inlet temperature (12.2 °C–23 °C). Performance characteristics of the GEHP were characterized by water outlet temperatures, cooling capacity, heating capacity and primary energy ratio (PER). The results showed that the effect of evaporator water inlet temperature on the system performance is more significant than the effects of ambient air temperature and evaporator water flow rate. PER of the considered system at evaporator water inlet temperature of 23 °C is higher than that one at evaporator water inlet temperature of 12.2 °C by about 22%. PER of the system decreases by 16% when engine speed changes from 1200 rpm to 1750 rpm.     

Simulation model of the thermal performance of a natural convection-type solar tunnel dryer

The basic objective of this paper is to develop a comprehensive simulation model of the thermal performance of solar tunnel dryer. The model is useful in system design as it is sensitive to the design parameters of air collector and dryer (like, length, radius, tunnel tilt, etc.). It is also useful in determining the drying behaviour of high-moisture products (vegetables, fruits, etc.) as well as low-moisture products (barely, wheat, etc.). The performance of the dryer has been estimated for a natural convection mode flow. A transient one-dimensional model was developed for the dryer and the numerical calculations were made for the climate of Delhi. It is observed that a large quantity of barley about 2135 kg can be dried in this dryer within two days of operation upto an equilibrium moisture content. © 1998 John Wiley & Sons Ltd.     

Influence of enhancing features on dehumidifier performance: Laboratory measurements

The influence of an evaporator economizer and liquid subcooler on the operating parameters and performance of a heat pump dehumidifier has been measured in a pschyrometic calorimeter under a range of operating conditions. The results show that the enhancement in drying capacity and efficiency is most significant at low relative humidity (RH). At RH = 25% the specific moisture extraction rate of the test system was increased by 50%, and the rate of condensate formation increased by 65%, when both the economizer and subcooler were used. These increases were limited by the thermal effectiveness of the economizer which was typically 25%. The effectiveness of the subcooler was approximately 75%.     

Thermal analysis of a new concept in a household clothes tumbler dryer

This paper presents a theoretical and experimental study of a novel water heat exchanger that heats the air in a domestic clothes tumbler dryer in place of a traditional electric heater, with a view to improve its energy efficiency. Modelling of a waste heat recovery heat exchanger has been undertaken using EES software package to assess its effectiveness on the drying cycle. The new dryer was found to have shorter drying times, better moisture extraction rates for the same power input and hence more efficient than the traditional dryer.     

Performance of indirect solar cabinet dryer

In this paper, the development and testing of a new type of efficient solar dryer, particularly meant for drying vegetables and fruit, is described. The dryer has two compartments: one for collecting solar radiation and producing thermal energy and the other for spreading the product to be dried. This arrangement was made to absorb maximum solar radiation by the absorber plate. In this dryer, the product was loaded beneath the absorber plate, which prevented the problem of discoloration due to irradiation by direct sunlight. Two axial flow fans, provided in the air inlet, can accelerate the drying rate. The dryer had six perforated trays for loading the material. The absorber plate of the dryer attained a temperature of 97.2 °C when it was studied under no load conditions. The maximum air temperature in the dryer, under this condition was 78.1 °C. The dryer was loaded with 4 kg of bitter gourd having an initial moisture content of 95%, and the final desired moisture content of 5% was achieved within 6 h without losing the product colour, while it was 11 h for open sun drying. The collector glazing was inclined at a particular angle, suitable to the location, for absorption of maximum solar radiation. A detailed performance analysis was done by three methods, namely ‘annualized cost method’, ‘present worth of annual savings’ and ‘present worth of cumulative savings’. The drying cost for 1 kg of bitter gourd was calculated as Rs. 17.52, and it was Rs. 41.35, in the case of an electric dryer. The life span of the solar dryer was assumed to be 20 years. The cumulative present worth of annual savings over the life of the solar dryer was calculated for bitter gourd drying, and it turned out be Rs. 31659.26, which was much higher than the capital cost of the dryer (Rs. 6500). The payback period was calculated as 3.26 years, which was also very small considering the life of the system (20 years). So, the dryer would dry products free of cost during almost its entire life span. The quality of the product dried in the solar dryer was competitive with the branded products available in the market.     

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.     

Wood chip drying with an absorption heat pump

This paper presents the thermal and economic analysis of a mobile wood chip drying process. The dryer was subjected to different operating conditions, which were studied in order to determine the optimal characteristics of the dryer in terms of energy consumption. In addition, the impact of the exterior climatic conditions on the dryer's performance was also evaluated. The performance of the dryer coupled with an absorption heat pump was modeled in steady-state conditions under different operating parameters. Finally the system's energy performance was compared to the performance of two other systems (a wood burning furnace and a waste-heat recovery system). The results demonstrate that single-stage absorption heat pumps are only interesting when the set point temperature of the drying air is below 60 °C. Otherwise, a two-stage absorption heat pump must be used. In terms of energy and financially, this type of drying is very costly. Of the three processes that were studied, heat recovery systems proved to be the most energy efficient and economic solution.     

Performance of a heat-pump assisted dryer

We present a simple mathematical model of a heat-pump-assisted dryer developed from psychrometric processes. A term ‘contact factor’ is used in the theoretical model to characterize the drying chamber. The experimental data of the drying rates of different types of products are used to predict the values of the contact factor of the dryers. We examine the effect of various parameters such as the contact factor, air inlet conditions, and the moisture removal rate on the performance of the heat-pump-assisted dryer. It has been shown that the non dimensional contact factor of a dryer is insensitive to dryer air inlet temperature. Finally, a performance chart to guide the selection of the heat-pump dryer components is proposed.