A heat pipe photovoltaic/thermal (PV/T) hybrid system and its performance evaluation

Building-integrated photovoltaic/thermal (BIPV/T) system has been considered as an attractive technology for building integration. The main part of a BIPV/T system is PV/T collector. In order to solve the non-uniform cooling of solar PV cells and control the operating temperature of solar PV cells conveniently, a heat pipe photovoltaic/thermal (PV/T) hybrid system (collector) has been proposed and described by selecting a wick heat pipe to absorb isothermally the excessive heat from solar PV cells. A theoretical model in terms of heat transfer process analysis in PV module panel and introducing the effectiveness-number of transfer unit (?-NTU) method in heat exchanger design was developed to predict the overall thermal-electrical conversion performances of the heat pipe PV/T system. A detailed parametric investigation by varying relevant parameters, i.e., inlet water temperature, water mass flow rate, packing factor of solar cell and heat loss coefficient has been carried out on the basis of the first and second laws of thermodynamics. Results show that the overall thermal, electrical and exergy efficiencies of the heat pipe PV/T hybrid system corresponding to 63.65%, 8.45% and 10.26%, respectively can be achieved under the operating conditions presented in this paper. The varying range of operating temperature for solar cell on the absorber plate is less than 2.5 °C. The heat pipe PV/T hybrid system is viable and exhibits the potential and competitiveness over the other conventional BIPV/T systems.     

Economic and exergy analysis of alternative plants for a zero carbon building complex

The feasibility of zero carbon emission plants for heating, air conditioning and domestic hot water (DHW) supply, is analyzed, with respect to conventional plants, for a new residential building complex to be constructed, in Northern Italy. Two zero carbon plants are considered: the first is composed of air-to-water heat pumps for space heating and cooling, PV solar collectors, air dehumidifiers, thermal solar collectors and a wood pellet boiler for DHW supply; in the second, the air-to-water heat pumps are replaced by ground-coupled heat pumps. The conventional plant is composed of a condensing gas boiler, single-apartment air to air heat pumps, and thermal solar collectors. The economic analysis shows that both zero carbon plants are feasible, and that the air-to air heat pumps yield a shorter payback time. The exergy analysis confirms the feasibility of both plants, and shows that the ground coupled heat pumps yield a higher exergy saving.     

Achieving total domestic hot water production with renewable energy

Various means of producing domestic hot water (DHW) with renewable energy in zero net energy homes (ZNEH) are examined for two climates (Montréal and Los Angeles). Four alternatives are examined: (i) a regular electric hot water tank; (ii) the desuperheater of a ground-source heat pump (GSHP) with electric backup; (iii) thermal solar collectors with electric backup; and (iv) a heat pump water heater (HPWH) indirectly coupled to a space conditioning GSHP. Results show that heating DHW with thermal solar collectors with an electric backup (which is either provided by the photovoltaic (PV) panels or the grid in a ZNEH) is the best solution for a ZNEH. The second part of this paper focuses on determining what should be the respective areas of the thermal solar collectors and PV array to obtain the least expensive solution to achieve total DHW production with renewable energy.     

Theoretical performance assessment of an integrated photovoltaic and earth air heat exchanger greenhouse using energy and exergy analysis methods

In this paper, a simplified mathematical model develops to study round the year effectiveness of photovoltaic/thermal (PV/T) and earth air heat exchanger (EAHE) integrated with a greenhouse, located at IIT Delhi, India. The solar energy application through photovoltaic system and earth air heat exchanger (EAHE) for heating and cooling of a greenhouse is studied with the help of this simplified mathematical model. Calculations are done for four types of weather conditions (a, b, c and d types) in New Delhi, India. The paper compares greenhouse air temperatures when it is operated with photovoltaic/thermal (PV/T) during daytime coupled with earth air heat exchanger (EAHE) at night, with air temperatures when it is operated exclusively with photovoltaic/thermal system (PV/T) and earth air heat exchanger (EAHE), for 24 h. The results reveal that air temperature inside the greenhouse can be increased by around 7-8 °C during winter season, when the system is operated with photovoltaic (PV/T), coupled with earth air heat exchanger (EAHE) at night. From the results, it is seen that the hourly useful thermal energy generated, during daytime and night, when the system is operated with photovoltaic (PV/T) coupled with earth air heat exchanger (EAHE), is 33 MJ and 24.5 MJ, respectively. The yearly thermal energy generated by the system has been calculated to be 24728.8 kWh, while the net electrical energy savings for the year is 805.9 kWh and the annual thermal exergy energy generated is 1006.2 kWh.     

Renewable energy for passive house heating: Model of the active solar heating system

The large windows on the south-oriented façade of a passive house strongly contribute to building space heating. These windows constitute the passive solar heating system. This paper studies the active heating system of a passive house, which includes the following sub-systems: (1) solar thermal collectors, (2) a water storage tank, (3) a secondary water circuit, (4) a domestic hot water preparation system and (5) an air ventilation and heating system. Models for all sub-systems are presented. The integrated model was implemented to Pirmasens Passive House (Rhineland Palatinate, Germany). The active solar heating system provides a smaller amount of heat than the heat provided by the passive solar heating system. Almost all the solar energy collected is not used for space heating but to domestic hot water (DHW) preparation. However, there is still a need for the classical water heater to operate all over the year. Almost all space heating thermal load is covered by using the classical air heater that operates mainly during the nights from November to April. The solar fraction lies between 0.180 in February and 0.679 in October, with a yearly average of 0.446. The study reveals that on a yearly basis it is more advantageous to use vertical south-oriented solar collectors instead of roof placed collectors.     

Effect of building integrated photovoltaics on microclimate of urban canopy layer

Building integrated photovoltaics (BIPV) has potential of becoming the mainstream of renewable energy in the urban environment. BIPV has significant influence on the thermal performance of building envelope and changes radiation energy balance by adding or replacing conventional building elements in urban areas. PTEBU model was developed to evaluate the effect of photovoltaic (PV) system on the microclimate of urban canopy layer. PTEBU model consists of four sub-models: PV thermal model, PV electrical performance model, building energy consumption model, and urban canyon energy budget model. PTEBU model is forced with temperature, wind speed, and solar radiation above the roof level and incorporates detailed data of PV system and urban canyon in Tianjin, China. The simulation results show that PV roof and PV façade with ventilated air gap significantly change the building surface temperature and sensible heat flux density, but the air temperature of urban canyon with PV module varies little compared with the urban canyon of no PV. The PV module also changes the magnitude and pattern of diurnal variation of the storage heat flux and the net radiation for the urban canyon with PV increase slightly. The increase in the PV conversion efficiency not only improves the PV power output, but also reduces the urban canyon air temperature.     

建筑一体化电热冷联产光伏组件能量特性研究

传统太阳能光伏或光热建筑一体化只能为建筑提供单一电能或热能。通过研究一种集成发电、集热、制冷3种功能的建筑一体化电热冷联产光伏组件,对其夏季工况下能量特性进行了实际检测。结果表明:白天,组件集热同时能有效降低光伏电池温度,组件工作温度高于环境温度约8~16℃,发电和集热效率分别为14.1%~13.7%和40.1%~15.7%;晴朗夜间,组件通过对流和辐射两种传热方式进行散热制冷,总制冷功率为26.0~268.5 W/m~2。电热冷联产光伏组件适合与热泵结合,为建筑提供所需能源。     

Long term operation of a solar assisted ground coupled heat pump system for space heating and domestic hot water

This paper introduces a solar-assisted ground-coupled heat pump (SAGCHP) system with heat storage for space heating and domestic hot water (DHW) supply. The simulation results of the system's detailed operating performance are presented. The optimization of the system design is carried out by the TRNSYS and a numerical simulation is performed for continuous operation of 20 years under the meteorological conditions of Beijing. Different control strategies are considered and the operational characteristics of each working mode are studied. The simulating results show that the long term yearly average space heating efficiency is improved by 26.3% compared to a traditional ground coupled heat pump (GCHP) system because the solar thermal collecting system is used to elevate the thermal energy in the soil and to provide direct space heating with heat storage. At the same time, the underground heat load imbalance problem for a heating load dominated GCHP is solved by soil recharging during non-heating periods, while extra solar energy is utilized to supply DHW. The flexibility and high efficiency of the SAGCHP system could offer an alternative for space heating and DHW supply by heat pump technology and solar energy in cold winters of northern China.     

Experimental and numerical investigation on thermal and electrical performance of a building integrated photovoltaic-thermal collector system

An experimentally validated computational fluid dynamics (CFD) model of a novel building integrated photovoltaic-thermal (BIPV/T) collector is studied to determine the effect of active heat recovery on cell efficiency and to determine the effectiveness of the device as a solar hot water heater. Parametric analysis indicates that cell efficiency can be raised by 5.3% and that water temperatures suitable for domestic hot water use are possible. Thermal and combined (thermal plus electrical) efficiencies reach 19% and 34.9%, respectively. A new correlation is developed relating electrical efficiency to collector inlet water temperature, ambient air temperature and insolation that allows cell efficiency to be calculated directly.     

Condenser heat recovery with a PV/T air heating collector to regenerate desiccant for reducing energy use of an air conditioning room

This paper presents an experimental test along with procedures to investigate the validity of a developed simulation model in predicting the dynamic performance of a condenser heat recovery with a photovoltaic/thermal (PV/T) air heating collector to regenerate desiccant for reducing energy use of an air conditioning room under the prevailing meteorological conditions in tropical climates. The system consists of five main parts; namely, living space, desiccant dehumidification and regeneration unit, air conditioning system, PV/T collector, and air mixing unit. The comparisons between the experimental results and the simulated results using the same meteorological data of the experiment show that the prediction results simulated by the model agree satisfactorily with those observed from the experiments. The thermal energy generated by the system can produce warm dry air as high as 53 °C and 23% relative humidity. Additionally, electricity of about 6% of the daily total solar radiation can be obtained from the PV/T collector in the system. Moreover, the use of a hybrid PV/T air heater, incorporated with the heat recovered from the condenser to regenerate the desiccant for dehumidification, can save the energy use of the air conditioning system by approximately 18%.     

Exergy efficiency analysis in buildings climatized with LiCl–H2O solar cooling systems that use swimming pools as heat sinks

Solar cooling is emerging as one of the most interesting applications in the harnessing of solar energy for alternative uses. Current devices can effectively control the climates of small buildings while addressing the issues associated with the excessive thermal energy captured during the summer months. This article presents an exergy analysis of buildings with solar thermal systems used for Domestic Hot Water (DHW) production and heating and cooling support. The cooling system analyzed is a LiCl–H₂O thermally driven heat pump with integral energy storage that uses outdoor swimming pools as heat sink. All subsystems were integrated into the model and considered as a single energy system, and data from installations in three different locations were used. The influences of the heating and cooling demand ratios and the dead state and house temperatures were analyzed. Further, the use of dissipated energy was analyzed, demonstrating that the proposed method facilitates the realistic study of these systems and provides useful analytical tools for improving the overall exergy performance. The energy delivered for heating, cooling and DHW production strongly influences global performance, suggesting that the appropriate sizing of each system is a priority.     

Impact of computational domain on the prediction of buoyancy-driven ventilation cooling

Traditional solar heated cavity structures such as solar chimneys make use of the stored solar energy in the interior wall to enhance natural ventilation of buildings but integration of photovoltaic devices into the exterior wall of such a structure can result in different proportions of heat distribution on both interior and exterior walls. This paper presents results of CFD simulation of the buoyancy-driven airflow and heat transfer in vertical cavities of different heights and widths with different total heat fluxes and wall heat distributions for ventilation cooling. Two sizes of computational domain were used for simulation – a small domain same as the physical size of a cavity and a large extended domain that is much larger than the cavity. The predicted natural ventilation rate and heat transfer coefficient have been found to depend on not only the cavity size and the quantity and proportion of heat distribution on the cavity walls but also the domain size. The difference in the predicted ventilation rate or heat transfer coefficient using the small and large domains is generally larger for wider cavities where heat distribution on two vertical walls is highly asymmetrical; incoming air would be distorted from symmetrical distribution across the inlet opening; and/or significant reverse flow would occur at the outlet opening. The difference in the heat transfer coefficient is generally less than that in the ventilation rate. In addition, a cavity with symmetrical heating has a higher ventilation rate but lower heat transfer coefficient than does an asymmetrically heated cavity.     

Thermal physiological response to local heating and cooling during sleep

For a healthy and productive life, good sleep is essential, which has prompted studies on how comfortable sleep can be achieved. Understanding the relationship between thermal environment and physiological responses such as skin and core temperatures, and psychological responses such as thermal and sleep sensations is necessary to identify the most suitable thermal environment for sleep. As an energy-saving and practical method of creating the most appropriate thermal environment for sleep, local heating or cooling is sometimes used, which takes into consideration the differences in local thermal responses. We performed this study to identify the most effective thermal environment for inducing comfortable sleep by identifying the physiological responses during sleep on the basis of sleep experiments conducted under local body heating or cooling conditions. We also used a human thermal model, which can be applied for predicting physiological responses.In the experiments, the feet of the subject were the primary area to be heated or cooled, which was achieved by installing a flexible duct with an outlet placed close to the subject's feet and inlet connected to an air conditioner. Differences in the fluctuation of body temperature and sleep stage depended on the airflow direction from the duct to the feet. When air was blown downward towards the feet, body temperature decreased and the subject was able to sleep well. Measured skin and core temperatures were calculated using an improved 27-node human thermal model that was originally developed for use in subjects who are not in sleep. Although skin temperature fluctuated significantly under local cooling, the results calculated using the proposed model agreed well with the measured results since the changes in heat conductance between the skin surface and surrounding environment as a result of the changes in the posture and feet position were taken into account. This result indicates that posture-associated changes in the heat conductance significantly influence skin temperature.     

Numerical simulation of natural ventilation of a factory roof cavity

The study targets the reduction of roof solar heat gain through the use of natural ventilation in a cavity of a factory roof. In the laboratory experiment [1], the average air velocity reached 0.25 m/s. A simulation program was developed to calculate the heat and air flow attained in the experiment. An airflow passage was divided into sections to trace the pattern of the air temperature rise. When the cavity was divided into 20 sections, it was enough to trace the temperature rise pattern, and hence to calculate buoyancy for natural ventilation. Then the simulated air velocities, temperatures and heat transportations were compared with the experimental results. The molecular viscosity and thermal conductivity of the air were modified to adjust the simulation results to the experimental results in a wide range of experimental conditions. When they were multiplied with a magnitude of 30 equally, the least root mean square of the ratio of deviations of the heat transportation was obtained. This simulation could predict the heat transportation as a result of natural ventilation with a root mean square of the deviation of 0.25 in a short calculation time.     

Energy and exergy analysis of PV/T air collectors connected in series

In this paper an attempt has been made to derive the analytical expressions for N hybrid photovoltaic/thermal (PV/T) air collectors connected in series. The performance of collectors is evaluated by considering the two different cases, namely, Case I (air collector is fully covered by PV module (glass to glass) and air flows above the absorber plate) and Case II (air collector is fully covered by PV module (glass to glass) and air flows below the absorber plate). This paper shows the detailed analysis of energy, exergy and electrical energy by varying the number of collectors and air velocity considering four weather conditions (a, b, c and d type) and five different cities (New Delhi, Bangalore, Mumbai, Srinagar, and Jodhpur) of India. It is found that the collectors fully covered by PV module and air flows below the absorber plate gives better results in terms of thermal energy, electrical energy and exergy gain. Physical implementation of BIPV system has also been evaluated. If this type of system is installed on roof of building or integrated with building envelope will simultaneously fulfill the electricity generation for lighting purpose and hot air can be used for space heating or drying.     

光伏光热建筑一体化对建筑节能影响的理论研究

对光伏光热建筑一体化(BIPV／T)的两种主要模式建立了理论模型，采用香港地区典型年的气象数据对两种BIPV／T模式的热性能进行了计算分析。与常规建筑相比，光伏光热建筑减少了墙体得热，改善了室内空调负荷状况，提高了建筑节能效果。     

Unsteady thermal performance analysis of a room with serial and parallel duct radiant floor heating system using hot airflow

In this study, the unsteady thermal performance of a test room heated by circulating hot airflow under the floor was analyzed with a developed mathematical model based on heat transfer equilibrium among the air flow, the floor and the indoor air. The time variations in the indoor air temperature for the serial duct floor heating system were investigated theoretically and experimentally. The time variations in the floor surface and the indoor air temperatures were predicted theoretically for the parallel duct floor heating system. Experiments on the time variations of the dimensionless numbers such as Nu for the airflow in duct and the indoor air, Gr for the indoor air and the heat ratios of convection and radiation to total heat for the serial duct floor heating system were performed. The theoretical and experimental results showed a good agreement.     

Field performance of a Japanese low energy home relying on renewable energy

This paper describes the construction and evaluation of an experimental low energy home assisted by a hybrid system using natural energy resources and unused energy. The home, for which a ground source heat pump (GSHP) system has been installed, was built on the campus of Hokkaido University, Japan in March 1997. The total floor area of the home is 192 m². This home is super insulated and airtight; the calculated coefficient of heat loss is 0.97 W/m² K. It has various passive strategies including direct solar heat gain and a ventilation system with an exhaust stack. Photovoltaic (PV) modules, wind power and solar collectors are adopted in order to achieve self-sufficiency in electric power and domestic hot water (DHW) supply. A GSHP is used for space heating and cooling. Two vertical steel wells are used as vertical earth heat exchangers (VHE). In summer, there is a floor cooling system using piped cold water from the VHE.Approximately 80% of the home’s total energy was provided by PV modules, solar collectors, as well as underground and exhaust heat. The annual amount of purchased energy during the test period was 12.5% that of a typical home in Hokkaido.     

Active thermal insulators: Finite elements modeling and parametric study of thermoelectric modules integrated into a double pane glazing system

Active thermal insulators (ATI) represent a new thermal control technology that uses solar energy to compensate for the passive heat losses or gains in building envelopes. This effect is accomplished by integrating photovoltaic (PV) and thermoelectric (TE) systems into a wall assembly. A parametric study is presented considering the implementation of TE-modules into the air cavity of a double pane glazing system. The main objectives of this study are to explore design configurations that maximize the coefficient of performance (COP) of the TE-heat pump units, and maximize the ability of the system to perform as a heating and cooling system for use in buildings. A finite elements model (FEM) was developed and experimentally validated to calculate the steady-state heat transfer for the ATI-system. A parametric study was undertaken to determine: (i) suitable TE-modules for this application, (ii) the optimal spreading of the TE-heat pumps, and (iii) the composition of the double glazing unit. The results of our study indicate that the system can be properly designed for heating purposes, however, a more optimal design will need to be realized in order to make the approach effective for cooling applications.     

Exergetic performance assessment of a solar photovoltaic thermal (PV/T) air collector

In this paper, an attempt is made to evaluate the exergetic performance of a solar photovoltaic thermal (PV/T) air collector. A detailed energy and exergy analysis is carried out to calculate the thermal and electrical parameters, exergy components and exergy efficiency of a typical PV/T air collector. Some corrections are done on related heat loss coefficients. An improved electrical model is used to estimate the electrical parameters of a PV/T air collector. Further, a modified equation for the exergy efficiency of a PV/T air collector is derived in terms of design and climatic parameters. A computer simulation program is also developed to calculate the thermal and electrical parameters of a PV/T air collector. The results of numerical simulation are in good agreement with the experimental measurements noted in the previous literature. Finally, parametric studies have been carried out. It is observed that the modified exergy efficiency obtained in this paper is in good agreement with the one given by the previous literature. It is also found that the thermal efficiency, electrical efficiency, overall energy efficiency and exergy efficiency of PV/T air collector is about 17.18%, 10.01%, 45% and 10.75% respectively for a sample climatic, operating and design parameters.