Exhaust Air Heat Pump Research Articles

Performance analysis of a fresh air and indoor air co-treatment parallel-loop exhaust air heat pump for buildings

To reduce building energy consumption and solve the problem that the power of traditional exhaust air heat pump is limited by exhaust air volume, an innovative fresh air and indoor air co-treatment parallel-loop exhaust air heat pump (CPEAHP) was proposed, and three operating modes of the CPEAHP were studied. The results showed that the co-treatment mode not only increased the flexibility of system regulation, but also improved the heating performance of the system at low temperature in winter. When the winter outdoor temperature was higher than 0 °C, the temperature efficiency of the co-treatment mode was higher than 100 %, and that efficiency was higher than 100 % under all outdoor temperature conditions in summer. At a winter outdoor temperature of −15 °C, the co-treatment mode could increase the temperature of fresh air by 20.4 °C and increase the temperature of indoor air by 3.1 °C, the heating capacity of the system was 6.3 kW and the COP was 8.7 at this time. At an outdoor temperature of 42 °C in summer, the co-treatment mode could decrease the temperature of fresh air by 22.8 °C and decrease the temperature of indoor air by 3.1 °C, the cooling capacity of the system was 7.3 kW and the COP was 4.5 at this time. These results demonstrated the excellent performance of CPEAHP system, and the application of CPEAHP system was expected to reduce building energy consumption and promote carbon neutrality in the construction industry.

Greenhouse Gas Payback Time of Different HVAC Systems in the Renovation of Nordic District-Heated Multifamily Buildings Considering Future Energy Production Scenarios

The European Union (EU) has implemented several policies to enhance energy efficiency. Among these policies is the objective of achieving energy-efficient renovations in at least 3% of EU buildings annually. The primary aim of this study was to offer a precise environmental comparison among four similar district-heated multifamily buildings that have undergone identical energy efficiency measures. The key distinguishing factor among them lies in the HVAC systems installed. The chosen systems were as follows: (1) exhaust ventilation with air pressure control; (2) mechanical ventilation with heat recovery; (3) exhaust ventilation with an exhaust air heat pump; and (4) exhaust ventilation with an exhaust air heat pump with a Photovoltaic (PV) panel. This study involved a life cycle assessment that relied on actual material data from the housing company and energy consumption measurements. This study covered a period of 50 years for thorough analysis. A sensitivity analysis was also conducted to account for various future scenarios of energy production. The findings revealed that the building with an exhaust air heat pump exhibited the lowest greenhouse gas emissions and the shortest carbon payback period (GBPT), needing only around 7 years. In contrast, the building with exhaust ventilation without heat recovery showed the highest emissions and the longest carbon payback period (GBPT), requiring approximately 11 years. Notably, the results were significantly influenced by future scenarios of energy production, emphasizing the crucial role of emission factors in determining the environmental performance of distinct renovation scenarios.

The thermal performance and applicability analysis of the composite ventilation system with heat recovery in ultra-low energy buildings

By analyzing the energy transfer process of building ventilators with heat recovery, a novel building ventilator with exhaust air heat pump enhanced by pump-driven loop heat pipe system was proposed and experimentally compared with the dual-loop heat pump and traditional heat pump system. During the test, the thermal performance of the prototype under three different operating modes was comprehensively studied, and then the energy-saving potential and applicability of the heat recovery units in the ultra-low energy buildings (ULEBs) were analyzed by using the DEST and MATLAB tools. Results showed that compared with the heat pump system, the composite system presented the highest energy efficiency ratio (EER) in the whole year, at 12.6 for winter and 4.8 for summer conditions, respectively. What's more, the energy recovered by the composite system is in line with the fresh air load, which can solve the problem of energy recovery attenuation of traditional ventilators in cold climate conditions. In addition, the composite system had great applicability in ULEBs, and the energy-saving ratio was 35.3% and 14.5% in Harbin and Kunming, respectively. Future works are to improve the performance of the pump-driven loop heat pipe and heat pump unit under all operating conditions for better adaptability of the ventilators.

Integrated system of exhaust air heat pump and advanced air distribution for energy-efficient provision of outdoor air.

A large outdoor air supply is required to control the airborne infection risk of respiratory diseases (e.g., COVID 19) but causes a high energy penalty. This study proposes a novel integrated system of the exhaust air heat pump and advanced air distribution to energy-efficiently provide outdoor air. The system energy performances are evaluated by the experimentally validated thermodynamic model of heat pump and heat removal efficiency model of advanced air distribution. Results show the exhaust air heat pump with advanced air distribution can save energy because of three mechanisms. First, the exhaust air heat pump reuses the exhaust air to reduce the condensation temperature, thereby improving the coefficient of performance. Second, advanced air distribution reduces ventilation load. Third, advanced air distribution reduces the condensation temperature and enhances the evaporation temperature, thereby improving the coefficient of performance. The exhaust air heat pump saves energy by 18%, advanced air distribution saves energy by 36%, and the integrated system of the exhaust air heat pump and advanced air distribution can save energy by 45%. As a specific application, compared with the conventional system (i.e., the outdoor air heat pump with mixing ventilation), the exhaust air heat pump with stratum ventilation saves energy by 21% − 35% under various outdoor air ratios and outdoor air temperatures. The proposed integrated system of the exhaust air heat pump and advanced air distribution contributes to the development of low-carbon and healthy buildings.

The applicability and energy consumption of a parallel-loop exhaust air heat pump for environment control in ultra-low energy building

• A novel inverter driven dual-cylinder rotary compressor was used. • The energy efficient working mode was proposed. • The system can well satisfy the requirements of building. A novel dual-cylinder rotary compressor was designed to improve the indoor space utilization, and it was used to drive a parallel-loop exhaust air heat pump (PEAHP) which integrates the functions of heating, cooling, ventilation and heat recovery. The operation characteristics of the system in different compressor frequencies and different fresh-air-to-return-air ratios (FRRs) were studied and analyzed experimentally. Through the simulation of the ultra-low energy buildings (ULEBs) in 12 cold zone cities, the hourly (8760 h) building thermal loads in the whole year are calculated. On the basis of the matching of the hourly system working characteristics and the thermal loads of ULEB, the energy efficient working strategy is proposed. Finally, the applicability and energy consumption of the PEAHP system applying in ULEBs were analyzed. The results show that, the system heating/cooling capacity and COP can be considerably improved by increasing return air volume flow rate. In winter, under the working condition of 50 Hz-FRR 1:1, the peak heating capacity and COP are 8.94 kW and 9.63, respectively, which are 77.38% and 64.05% higher than those when FRR is 1:0. In summer, when the outdoor temperature ranges from 26 °C to 40 °C, the system temperature effectiveness is higher than 1 in all working modes. The PEAHP system has great applicability for ULEBs in cold zone of China during winter, and the system can meet the year-round use demands of the ULEBs in Taiyuan, Dalian, Lanzhou, Yinchuan and Lhasa, with the unsatisfied heating/cooling time ratio of 1.61%, 0.09%, 0.006%, 2.45% and 0.02%, respectively.

High-efficiency cooling solution for exhaust air heat pump: Modeling and experimental validation

The cooling performance of exhaust air heat pumps is limited by the high condensing temperatures owing to insufficient exhaust air volume. In this study, a high-efficiency solution for an exhaust air heat pump was developed and numerically studied. In this novel system, the exhaust air is stepped energy recovered by an indirect evaporative cooler and a heat pump with an evaporative condenser. It is considered that the combination of these two heat transfer components can provide a system with a significantly high cooling efficiency. To explore the application potential of this system further, a detailed mathematical model was established. Meanwhile, a prototype based on this design was developed, and its performance in four operating modes was tested for model validation. When the test results are compared with those of a conventional system of the same size, they show that the cooling capacity and energy efficiency ratio of the novel system increased by 53% and 135%, respectively. Subsequently, to clarify the principles underlying the design of this system, parameter sensitivity analysis and system matching optimization were conducted numerically.

Waste heat recovery potential in residential apartment buildings in Finland's Kymenlaakso region by using mechanical exhaust air ventilation and heat pumps

With the growing share of ventilation heat loads, the heat recovery over the mechanical ventilation systems appears as one of the key solutions to reduce heat loss and generate energy savings. Finland's long-term renovation strategy aims for a highly energy-efficient and almost carbon-free building stock by 2050. One of the greatest potentials for energy savings relates to those built between the 1960s and 1980s where no heat recovery exists. This study addresses the wasted heat potential of apartment buildings in the Kymenlaakso region in Finland and how EAHPs can utilize it. The wasted heat potential was mapped by selecting buildings of different ages in Kotka from the 1950s to 2010 and for which the thermal energy balance was determined. The apartment buildings with mechanical exhaust ventilation in Kymenlaakso were surveyed. If all the mapped exhaust air waste energy could be utilized by EAHPs, it could produce the thermal energy combined from different decades up to 18.7 GWh/a in Kotka and 36.8 GWh/a in Kouvola. The calculated annual reduction in CO 2 emissions might be about 590 and 944 tCO 2 in Kotka and Kouvola, respectively. The calculated payback periods varied from 7 to 13 years for the selected buildings.

Technical requirements analysis of integrated paralleled-loop exhaust air heat pump system applied to ultra-low energy building in different climatic regions of China

Based on the simulation results of the typical rural ultra-low energy building (ULEB) in five different climatic regions of China, three indicative technical parameters for paralleled-loop exhaust air heat pump (PEAHP) R&D which are nominal heating-cooling capacity, maximum required fresh air to return air ratio (MFRR) and system energy efficiency grades were calculated and summarized according to the demand of indoor thermal comforts by using statistic method. The nominal heating-cooling capacities were determined according to the peak loads, which are 6.84-2.01 kW, 5-2.96 kW, 3.9-4.6 kW, 3.08-5.02 kW, and 3.4-0.46 kW in the ULEB of Harbin, Beijing, Shanghai, Guangzhou, and Kunming, respectively. To ensure both thermal comforts and energy conservation, during the heating season, full fresh air supply is suggested in Beijing and the 1:0.5 MFRR is suggested in Harbin, Shanghai, Guangzhou, and Kunming. During the cooling season, the 1:5 MFRR is suggested in Shanghai and Guangzhou, the 1:3, 1:1.5, and 1:0.5 MFRR are suggested in Harbin, Beijing and Kunming, respectively. The PEAHP energy efficiency grades 1~5 are 7.92~11.7, 7.58~11.5, 7.5~11.35, 6.12~9.27, and 4.64~7.03 during the heating season of Harbin, Beijing, Shanghai, Guangzhou, and Kunming, respectively, and are 2.33~3.54, 3.93~5.96, 4.61~6.98, 4.62~6.99, and 2.04~3.1 for the cooling season, respectively.

Experimental study and operation optimization of a parallel-loop heat pump for exhaust air recovery in residential buildings

The indoor space of residential building is limited, especially in building retrofit, to improve the indoor space utilization and the performance of exhaust air heat recovery system, a novel dual-cylinder rotary compressor with independent suction and discharge ports was designed to drive an inverter parallel-loop exhaust air heat pump. Through the experiments and analyzing the parallel-loop exhaust air heat pump system working characteristics under variable compressor frequency and different fresh air volume flow rate, the optimized method is proposed and verified experimentally. Finally, the annual energy efficient operation plan was proposed, and the seasonal performance factor and seasonal average input power under this operation plan in three typical countries/organization were calculated. Results show that the system COP can be greatly improved by applying a mixed air supply method during winter. When the fresh-air-to-return-air ratio is 1:1 and the outdoor temperature is −5 °C, the system COP reached 10.18, which is the highest comparing with the related studies. In summer, the system COP is increased by increasing the ventilation volume flow rate. When the operation of the parallel-loop exhaust air heat pump system follows the energy efficient operation plan to ensure the temperature effectiveness over 100% and the outdoor working condition complies with GB 21455–2013, the heating season performance factor, heating season average input power, cooling season performance factor, and cooling season average input power are 7.22, 0.6909 kW, 2.83, and 0.8167 kW, respectively. The work would provide guidance for the development of exhaust air heat recovery system in residential building.

Techno-economic analysis of an exhaust air heat pump system assisted by unglazed transpired solar collectors in a Swedish residential cluster

Solar heating technologies hold a significant potential to supplement or replace the fossil fuel-driven heating systems in residential and industrial applications. This paper presents a techno-economic study aiming to assess the use of Unglazed Transpired Solar Collectors (UTSC) coupled with an energy system assisted by Exhaust Air Heat Pump (EAHP) in cold climates applied to a residential building cluster. The performance of the system and its components is assessed for different sizes of solar collector field. In addition, a rule-based algorithm is developed to manage the airflow into the UTSC, and a comparative analysis is carried out with conventional flow control. The existing EAHP assisted energy system of a multifamily building cluster in Sweden is modelled by using a simulation software TRNSYS, and the effects of the UTSCs integration on the performances of the energy system are evaluated. Results show that the integration of UTSCs has a small but positive impact on the overall system performance. Moreover, the developed control based on the variation of the collector airflow rate for UTSC is an effective control strategy to increase the seasonal performance factor of the overall system and to maximize the savings.

Experimental and analytical evaluation of exhaust air heat pumps in ventilation-based heating systems

The application of exhaust air heat pumps as an energy efficient heating technology to cover the heating and ventilation demand of highly efficient residential buildings is becoming popular. Available studies on exhaust air heat pumps tend mostly to focus only on comparing different technologies in efficient buildings. Most of the existing studies ignore the usual presence of the electrical heaters as backup. Moreover, the impact of varying boundary conditions on the heat pump's performance is often not considered in depth. In this sense, there is still a need for discussion on the influence of different buildings' standards and control strategies on heating performance. The present paper aims to consider the energy efficiency of exhaust air heat pumps under different operating conditions. In this study, the results of a long-term field monitoring are utilized to model the dynamic behavior of an exhaust air heat pump in MATLAB/Simulink using black box modeling approach. The impact of different boundary conditions such as outdoor and exhaust air conditions on the heat pump's efficiency is studied and additionally compared to other studies. Moreover, a physical approach to model the defrosting process of the evaporator was developed and integrated into the black-box model; to use annual simulations of ventilation-based heating systems in three different building standards. Finally, the influence of five different control strategies on system performance is investigated. The core findings of this study reveal that: (1) Due to the limited exhaust air volume flow rate the impact of the exhaust air moisture content with its condensation enthalpy on the HP performance is significant. (2) In designing ventilation-based heating systems with exhaust air heat pumps due to their limited power (e.g. 6 Wh/m³ at A-2), the control strategy of backup electrical heaters must be selected carefully. (3) Conventional heating control methods such as Nighttime Temperature Reduction (NTR) following a reference room are hardly suitable in ventilation-based heating systems. (4) However, employing the deployed control strategy, the electrical energy consumption of the system could be reduced up to 40% compared to the conventional heating control methods.

Study of novel solar assisted heating system

The potential for energy, carbon dioxide equivalent (CO2e) and cost savings when using low emissivity (low-ε) transpired solar collectors (TSCs), combined with heat pumps in a range of configurations, has been investigated using computer modelling. Low-ε TSCs consist of metal solar collector plates with a spectrally sensitive surface, perforated with holes. Ambient air is drawn through the holes and heated by convection from the solar collector plate, increasing the air temperature by up to 25 K. The heated air can be used for e.g. space heating, or pre-heating water in buildings. The models developed have been used to compare the performance of low-ε TSC/heat pump heating systems in small and large buildings, at a range of locations. The model results showed savings in energy, CO2e and costs of up to 16.4% when using low-ε TSCs combined with an exhaust air heat pump compared with using the exhaust air heat pump alone. Practical application: If the UK is to meet its target of reaching net zero greenhouse gas emissions by 2050, it will be necessary to adopt low or zero carbon heating technologies. The novel low emissivity transpired solar collector device investigated can contribute to this. Its advantages include: (i) utilising solar radiation; (ii) readily integrated with existing heating systems e.g. heat pumps; (iii) significant energy, CO2e emissions and cost savings; (iv) low cost device; (v) minimal energy input i.e. one small fan; (vi) can be retrofitted to existing buildings; (vii) its benefits were applicable at all of the (wide range of) locations tested.

Performance comparison of heat recovery systems to reduce viral contagion in indoor environments.

Strong ventilation increments are currently suggested for containing the airborne diffusion of COVID-19 in indoor environments. However, it can involve an unacceptable growing of energy consumption. Therefore, maximum care must be addressed to improve efficiency of ventilation heat recovery (VHR). For this purpose, this paper investigates the opportunity of a technical solution. Consisting in adding downstream of the most diffuse heat recuperator, a heat pump using exhaust air as a cold source. An autonomous high efficiency air handling unit (HEAHU) was modelled for a school application. By simulation a performance comparison was carried on with two alternative systems based only on an exhaust air heat pump (EAHP) or on a heat recuperator for different weather conditions. Results indicated that the milder climate strongly penalizes heat recuperator and this fact deeply influences the conclusions. HEAHU saving compared to energy consumption of only heat recuperator is between 31% and 46%. For EAHP this saving varies from 2.5% to 48%. Only with a milder climate, EAHP presents a lightly greater saving than HEAHU. Heat pump technology looks to be very performing to foster the efficiency of VHR, especially in presence of high ventilation rates.

Experimental study of a novel household exhaust air heat pump enhanced by indirect evaporative cooling

This paper presents an experimental study of a novel household ventilator with exhaust air heat pump enhanced by indirect evaporative cooling. In this study, a prototype was made according to this theory and tested on the test rig. The test was carried out to evaluate the energy performance of the system during the cooling season. During the test, the thermal performances of the prototype under four different operating modes were comprehensively studied. In the typical summer conditions, the prototype operating as an indirect evaporative cooler followed by an evaporative condenser heat pump shows the highest cooling capacity at 4.27 kW and energy efficiency ratio (EER) at 5.12, which is a great performance improvement compared with that of conventional operating methods. The results of variable conditions tests show that this system can operate efficiently under different summer ambient conditions with the EER varying from 4.16 to 5.65. Besides, a comparison with other prototypes found in existing literature was also made in this study. Give consideration to compact and energy efficiency, the novel ventilation system shows a good academic value and application prospect in energy saving air-conditioning domain.

Evaluation of the theoretical and in-use performance of Exhaust Air Heat Pumps

Integrated units that combine Exhaust Air Heat Pumps (EAHPs) with Heat Recovery Ventilation (HRV) and, in some instances, Domestic Hot Water Storage (DHW), are becoming increasingly popular in the domestic market across Europe with over 24,000 EAHPs purchased in the EU in 2017 alone. Early research into using EAHPs demonstrated energy savings being conservatively between 20% and 50% when compared to conventional systems. Recent research has suggested that, in reality, EAHPs in-use energy performance can be worse than that estimated by various standardised theoretical assessment methods (COP/SPF in the range of 0.4 to 6.0). More worryingly, published data on this in-use operation is effectively non-existent for NZEB type dwellings and few studies have stress tested the robustness of the EN standards in accounting for the effects of in-use operation. The study presented in this paper investigated whether the standard methods used to predict in-use energy performance are sufficiently robust and adequately capture operational performance for EAHP systems. The energy performance of two identical EAHP systems in Ireland (one rural/ one urban) were monitored for close to 12 months. During the live in-use monitoring period, the EAHPs had ‘heat-pump/heat recovery only’ operating mode ratios of 16%/84% and 22%/77% for rural and urban systems respectively. The average HRE in-use efficiency was 92% and 64% for the rural and urban systems respectively. While the manufacturers stated Seasonal performance factors (SPF) ranging from 2.2 (for DHW) to 5.8 (for Space Heating), the average in-use SPF was found to vary between 1.7 and 3.8 depending on the boundary reported and the location. More research is urgently required in order to bring much needed clarity for designers and energy assessors regarding which boundaries can be universally applied to EAHP systems. Given the range of SPF which could apply to the HP’s examined, the paper highlights the importance of ensuring that realistic indicators of in use performance are provided, aiding appropriate decision-making by policymakers, industry and end-users.

Measured performance of exhaust air heat pumps in Finnish apartment buildings

The energy efficiency of existing apartment buildings is playing an important role in energy and climate targets. In Finland, mechanical exhaust ventilation system is commonly used in older apartment buildings. Hence, there could be an energy saving potential by an exhaust air heat pump system (EAHP). In this work two cases have been studied. Buildings were built in 1960’s and 1970’s and in renovation equipped with hybrid heating system: district heating and exhaust air heat pump system. Two years measurement data, 2018 and 2019, was collected to evaluate the performance of exhaust air heat pump systems. According to measurement data the monthly coefficient of performance (COP) was calculated as well as seasonal coefficient of performance (SCOP) was defined. The monthly COP values varied from 3,1 to 4,6 and SCOP values were about 3,7. Heating energy cost savings were 23-31 %. Energy performance class before and after EAHP installation was calculated. If at least 50 % of heating energy consumption was covered by EAHP then also energy performance class was improved.

Seasonal storage of residential exhaust air and sewage waste heat

Most Finnish residential buildings have been built before ventilation heat recovery options became mandatory. Exhaust air heat pumps are an effective way to reduce emissions, but they cannot cover all heating demand. Ground-source heat pumps can be designed to meet all loads, but they require corresponding amounts of space both above and below ground. This simulation study combines residential ventilation and sewage waste heat with a ground-source heat pump system to improve system sustainability and cost-effectiveness. A hybrid waste heat and ground-source heat pump system was shown to have 20% lower life cycle costs compared to a pure ground-source heat pump system. It also maintained sustainable ground temperature levels over the long term, while reducing above-ground space requirements by 95%.

Study on waste heat recoveries and energy saving in combined energy system of ice and swimming halls in Finland

Ice and swimming halls annually consume lots of energy and produce significant amount of potential waste heat in Finland. In this paper, utilization of four possible waste heat sources (ice refrigeration, dehumidification of air, Gray water and exhaust air) is studied by simulating combined energy system of ice and swimming halls locating in Helsinki. Four cases were simulated including the reference case, one case with waste heat recovery and two cases with both waste heat recovery and two different exhaust air heat pumps. In addition, thermal energy storage tanks are used to store the excess waste heat from the ice hall, while the high temperature exhaust air heat pumps can raise the waste heat temperatures for all heat demands. The results show that up to 99% of the purchased district heat can be replaced by the waste heat in the ice hall at the cost of only 9% purchased electricity increase. The combined utilization of excess heat transferred from the ice hall and the waste heat from the swimming hall can result in 72% reduction of purchased district heat and 37% electricity demand increase in the swimming hall. In the combined energy system of the studied ice and swimming hall, altogether 77% waste heat is utilized, bringing in 82% purchased district heat decrease and 25% electricity increase, while the total consumed energy reduced by 42%. In addition, the total annual energy cost savings reach 133 k€ (-29%), while the saving of the energy cost of the combined system can make up the maximum cost of the profitable investment. During three repayment periods (7, 10 and 15 years), the energy cost savings and maximum cost of profitable investment for the ice hall alone and combined ice and swimming halls are between 510 k€ and 970 k€ and between 700 k€ and 1 580 k€, respectively.

The Influence of Building Renovations on Indoor Comfort—A Field Test in an Apartment Building

This article presents a field test of how deep renovation affects indoor climate quality. The studied apartment building was built in 1968 and is located in Finland, within the Nordic climate zone. The deep renovation included façade repair with extra insulation, new windows with trickle vents, new balcony glass and doors, and the installation of an exhaust air heat pump into the existing mechanical exhaust air ventilation. The indoor climate conditions and building envelope tightness were measured before and after the renovation. As a result of these energy renovation measures, the building envelope tightness improved by nearly 40% and the uncontrolled supply of air (draughts) decreased by approximately 24%. The overall energy consumption of the building decreased by 45%. Above all, the long testing period gives credibility to the study. The field test brought up the challenge of supplying an adequate amount of fresh air. This article highlights the fact that windows are part of a mechanical ventilation system if fresh air is not controlled by being led through inlet ducts. The supply air flow and volume must be ensured by correctly dimensioned valves, and therefore we stress the importance of the technical cooperation of technical designers.

A novel packaged outdoor air dehumidifier with exhaust air heat pump – Experiment and simulation

Outdoor air dehumidification is one of the key functions of the ventilation system. A small packaged ventilation system with a built-in exhaust air heat pump (EAHP) is becoming popular for residential use. However, the conventional EAHP has low dehumidification capability and energy efficiency because the restricted volume flow of exhaust air would lead to high condensing temperature. In this work, a novel EAHP with separation of subcooler and condenser is proposed for the packaged outdoor air dehumidifier. In the exhaust air duct, the return air first lowers the refrigerant liquid temperature in subcooler and then joins the bypass outdoor air to cool the condenser before exhaust. In this way, lower condensing temperature and lower liquid temperature before expansion valve can be achieved to improve both the dehumidification capability and energy efficiency. A functional prototype was designed and tested. It achieved the dehumidification capability of 78.2 kg/day and the cooling COP (coefficient of performance) of 3.65 under the rated conditions. Moreover, based on a validated model, simulations show that energy efficiency of the novel system is superior to that of the conventional ones under variable operating conditions except for low dehumidification load.