On the Development of the Solar Photovoltaic and Thermal (PVT) Collector

The solar photovoltaic and thermal (PVT) collector is a device which converts solar energy into thermal and electrical energies simultaneously. The PVT collector can be used whenever both electricity and hot water are required, for example, for domestic uses. It is a known fact that the efficiency of the solar (photovoltaic) cells decreases as operating temperatures increase. Therefore, a better and a more efficient use of these cells, calls for cooling the cells. One method for doing that is to use a heat exchange system, which cools the cells by means of a heat absorbing medium, such as water, flowing in pipes. The heat removed from the cells results in hot water. Another advantage of the PVT collector is its higher overall efficiency per unit area and lower packaging costs due to its compact design. In this paper a theoretical analysis of the PVT collector using a simulation model is presented. In this model the PVT collector is divided into a matrix of ``small' PVT collector units, each one consisting of several layers. The energy balance of each ``small' PVT collector unit is studied by analysis of the energies entering and leaving each one of its layers. Later, the process is applied to the PVT collector itself. A PVT collector was designed and constructed and putthru a series of experiments under varying load conditions, insolation levels and other climatological conditions.     

Performance evaluation of photovoltaic–thermosyphon system for subtropical climate application

The rapid development and sales volume of photovoltaic (PV) modules has created a promising business environment in the foreseeable future. However, the current electricity cost from PV is still several times higher than from the conventional power generation. One way to shorten the payback period is to bring in the hybrid photovoltaic–thermal (PVT) technology, which multiplies the energy outputs from the same collector surface area. In this paper, the performance evaluation of a new water-type PVT collector system is presented. The thermal collection making use of the thermosyphon principle eliminates the expense of pumping power. Experimental rigs were successfully built. A dynamic simulation model of the PVT collector system was developed and validated by the experimental measurements, together with two other similar models developed for PV module and solar hot-water collector. These were then used to predict the energy outputs and the payback periods for their applications in the subtropical climate, with Hong Kong as an example. The numerical results show that a payback period of 12 year for the PVT collector system is comparable to the side-by-side system, and is much shorter than the plain PV application. This is a great encouragement in marketing the PVT technology.     

Optimum performance of thermal trap collectors

The “thermal trap effect” in semitransparent material and the trapping system in the conventional flat-plate collectors with two, three or four glass or plastic covers with air-gaps in between are analysed under a common heading of “thermal trap collectors”. In general, a thermal trap collector consists of one or many slabs of semitransparent material of finite thickness with air-gaps in between, and an ideal withdrawal mechanism at the base of the trapping system to withdraw all available energy. This approach makes a comparative study of the two types of collectors possible, and provides data to design the appropriate withdrawal mechanism and operating conditions. A steady state analysis which neglects internal reflections and body radiation shows the existance of an optimum performance in single-layer thermal trap collectors and its dependence on thickness. A model which includes internal reflections is then analysed and the existance of the optimum performance and its dependence on thickness is demonstrated by taking the example of a single slab of methylmethacrylate plastic. The model is extended to multilayer thermal trap collectors and two examples are considered; a multilayer methylmethacrylate thermal trap collector and a multilayer “glass” thermal trap collector. The results show that the two-layer methyl methacrylate thermal trap collector has, in general, a better performance than the corresponding single or three and four-layer systems. But at high withdrawal efficiencies of about 60 per cent, the single layer methyl methacrylate shows its uniqueness and becomes competitive with the two-layer system. But the three and four-layer “glass” thermal trap collectors perform better than the corresponding single and two-layer ones, with the three-layer system having an overall better performance. These results show that the number of slabs in addition to thickness are important parameters in the study of the performance of thermal trap collectors.     

Design and optimization of solar industrial hot water systems with storage

This paper presents a new method for the design and optimization of solar industrial process hot water systems with storage. The single-pass open-loop design thermally “decouples” collectors from storage, hence insuring that collectors always heat the coldest fluid possible and that stored heat can be completely depleted by the nighttime load. So the single-pass open-loop design, in spite of the relatively low flow rates entailed, operates at higher system efficiency than conventional system designs. One solved example for an an industrial hot water application shows that the single-pass open-loop design delivers about 30 per cent more useful energy with roughly 30 per cent less storage than the conventional design. Moreover, storage tanks do not have to stand high pressures and can thus be significantly cheaper than in conventional systems. The effects of collector operating time, heat exchangers, and secondary system losses are also treated. The new method is extended to cover systems that require weekend storage. The introduction of weekend storage may be cost effective because it enables the designer to reduce collector area without reducing the yearly useful energy delivered by the system.     

Dimensioning of the solar heating system in the zero energy house in Denmark

The paper describes the project for a Zero Energy House constructed at the Technical University of Denmark. The house is designed and constructed in such a way that it can be heated all winter without any “artificial” energy supply, the main source being solar energy. With energy conservation arrangements, such as high-insulated constructions (30–40 cm mineral wool insulation), movable insulation of the windows and heat recovery in the ventilating system, the total heat requirement for space heating is calculated to 2300 kWh per year. For a typical, well insulated, one-storied, one-family house built in Denmark, the corresponding heat requirement is 20,000 kWh. The solar heating system is dimensioned to cover the heat requirements and the hot water supply for the Zero Energy House during the whole year on the basis of the weather data in the “Reference Year”. The solar heating system consists of a 42 m² flat-plate solar collector, a 30 m³ water storage tank (insulated with 60 cm of mineral wool), and a heat distribution system. A total heat balance is set up for the system and solved for each day of the “Reference Year”. Collected and accumulated solar energy in the system is about 7300 kWh per yr; 30 per cent of the collected energy is used for space heating, 30 per cent for hot water supply, and 40 per cent is heat loss from the accumulator tank. For the operation of the solar heating system, the pumps and valves need a conventional electric energy supply of 230 kWh per year (corresponding to 5 per cent of the useful solar energy).     

Side-by-side comparison of a pressurized and a nonpressurized solar water heating thermosiphon system

Two commercially available domestic size hot water heating systems, in which the natural convection maintained the flow of water from the collector to the tank, were studied experimentally under identical meteorological conditions. One of the systems was a pressurized type of system and the other was a nonpressurized type. The measurements have been validated by numerical calculations performed using a simplified theory. An explicit expression has been derived for the mass flow rate of water due to thermosiphon effect in terms of known physical parameters. The measurements show that for an incident solar energy of 6.75 kWh/m² of collector area, the useful energy available from the pressurized and nonpressurized system is 3.06 kWh and 3.83 kWh per unit collector area respectively yielding a daily average efficiency of 41% and 47%. The system's performance has also been evaluated for typical water consumptions in the domestic sector.     

Test results and evaluation of integrated collector storage systems with transparent insulation

Two integrated collector storage (ICS) prototypes with about 1 m² absorber surface each have been installed and investigated at the Institut für Solare Energiesysteme (ISE). Each consists of a water tank with an integrated collector, which is covered with a highly transparent insulating material and is very well insulated on the sides and at the back. During the test period from November 1986 to October 1987 one of them was operated at water-main pressure, and 40 liters corresponding to the hot water consumption of one person, were withdrawn every day. The solar fraction was 58% (68%) with a system efficiency of 28% (36%) (the values in parentheses also take the surplus energy in summer into account, see “Results”). The second one has been installed to study the stagnation (no water flow) performance during the same period. Detailed computer simulation programs have been developed and compared with the experimental results. Based on the u value and the transmittance-absorptance product (τα), the annual performance can be predicted with an accuracy of 4%. The influence of various parameters such as specific water withdrawal, angle of inclination, profile of water withdrawal, required hot water temperature etc. on the yearly solar fraction and yearly efficiency of the ICS can be studied. Some of the results are presented here.     

Long term performance of evacuated tubular solar water heaters in Sydney, Australia

The performance of a domestic hot water system employing evacuated tubular collectors is compared with two others employing flat plate collectors over a period of one year. The efficiency of the evacuated tubular collector system was about 1.8 times that of a non selective flat plate system and about 1.3 times that of a high quality selective flat plate system. The superior performance of the evacuated system is explained in terms of the distribution of incident energy as a function of (ΔT/G) for domestic systems and the normal incidence collector efficiency curves.     

Detailed analysis of the energy yield of systems with covered sheet-and-tube PVT collectors

Solar cells have a typical efficiency in the range of 5-20%, implying that 80% or more of the incident solar energy can be harvested in the form of heat and applied for low-temperature heating. In a PVT collector one tries to collect this heat. In this work, the electrical and thermal yield of solar domestic hot water systems with one-cover sheet-and-tube PVT collectors were considered. Objectives of the work were to understand the mechanisms determining these yields, to investigate measures to improve these yields and to investigate the yield consequences if various solar cell technologies are being used. The work was carried out using numerical simulations.A detailed quantitative understanding of all loss mechanisms was obtained, especially of those being inherent to the use of PVT collectors instead of PV modules and conventional thermal collectors. The annual electrical efficiencies of the PVT systems investigated were up to 14% (relative) lower compared to pure PV systems and the annual thermal efficiencies up to 19% (relative) lower compared to pure thermal collector systems. The loss of electrical efficiency is mainly caused by the relatively high fluid temperature. The loss of thermal efficiency is caused both by the high emissivity of the absorber and the withdrawal of electrical energy. However, both the loss of electrical and thermal efficiency can be reduced further by the application of anti-reflective coatings. The thermal efficiency can be improved by the application of a low-emissivity coating on the absorber, however at the cost of a reduced electrical efficiency.     

An experimental study of façade-integrated photovoltaic/water-heating system

The worldwide fast development of building-integrated solar technology has prompted the design alternatives of fixing the solar panels on the building façades. How to make full use of an integrative system to achieve the best energy performance can be an important area in the technology promotion. Hybrid solar system applying in buildings has the advantage of increasing the energy output per unit installed collector area. This paper describes an experimental study of a centralized photovoltaic and hot water collector wall system that can serve as a water pre-heating system. Collectors are mounted at vertical facades. Different operating modes were performed with measurements in different seasons. Natural water circulation was found more preferable than forced circulation in this hybrid solar collector system. The thermal efficiency was found 38.9% at zero reduced temperature, and the corresponding electricity conversion efficiency was 8.56%, during the late summer of Hong Kong. With the PVT wall, the space thermal loads can be much reduced both in summer and winter, leading to substantial energy savings. Suggestions were given on how to further improve the system performance.     

Technoeconomic optimization of solar natural convection hybrid hot water systems

In this paper a techno-economic model for a hybrid domestic hot water system operating under natural convection mode is presented. Three modes of auxiliary energy supply viz.

• A electric heater fitted in the solar hot water tank.
• B electric heater fitted in a small water tank in series with the solar hot water tank, and
• C an instant electric heater fitted in the tap

have been considered. Taking into account the life and the capital and maintenance costs of the solar and electrical equipments, the cost of useful energy (Rs/kWh) has been calculated for different values of the collector area and the tank capacity, and thereby the optimum collector area and tank capacity (for a given demand), corresponding to minimum cost of useful energy, has been determined. From numerical calculations made for the climate of Delhi, India (a representative composite climate) corresponding to the two cases of monthly hot water demand viz. (i) constant monthly demand, and (ii) variable monthly demand, it is seen that case (C) is the most economic design of the hybrid hot water system; numerical calculations have also been made for this case corresponding to the climates of Srinagar and Madras (representing cold and hot climates). The effect of government subsidy on the optimized values of collector area, tank capacity and cost of useful energy has also been investigated for the climate of Delhi.     

Plume entrainment effects in solar domestic hot water systems employing variable-flow-rate control strategies

Solar domestic hot water heating systems perform more efficiently if their storage tanks are perfectly thermally stratified. In real tanks, which do not perfectly stratify, the most important mechanism destroying stratification is plume entrainment. Plume entrainment occurs when cooler water is inserted into the tank top which contains hotter water. The resultant falling plume of cool water causes mixing. This paper uses computer simulation to evaluate and compare two strategies by which plume entrainment is minimized by controlling the collector flow rate. One strategy (calleá “SCOT”) maintains a constant collector outlet temperature, and the other (called “FCTR”) strategy maintains a constant temperature rise ΔT_set from inlet to outlet of the collector. The results of the study show that the SCOT strategy always produces a system that performs more poorly than the corresponding system with a fixed flow rate. The FCTR strategy, on the other hand, consistently out-performs the fixed flow strategy, but only by a few percent. When the FCTR strategy is used, the optimum ΔT_set to use is 20°C for the SDHW system simulated.     

Field performance and operation of a flat-glass solar heat collector

A “Base-Line,” flat-glass solar heat collector has been designed and constructed that can be manufactured economically for commercial use. Four of the collectors, 34 by 76 in. (approximately 18 ft²), were installed to provide hot water to a private home in Melbourne, Florida.The details of the collector are described, including coverplates, solar absorber, absorber coating, spacers, seals and glazing.A simple relationship has been established between the collector efficiency, the collector temperature and the rate of insolation for constant rates of flow of circulating fluids.The theoretical and field performance curves have been correlated for collector efficiency, collector temperatures, incident solar radiation and ambient air temperatures. The effect of fluid flow on collector temperatures for various collector parameters has also been presented.     

Negative buoyant plume model for solar domestic hot water tank systems incorporating a vertical inlet

Thermal stratification in solar energy storage tanks plays an important role in enhancing the performance of solar domestic hot water systems. The mixing that occurs when hot fluid from the solar collector enters the top of the tank is detrimental to the stratification. Mathematical models that are used for system analysis must therefore be able to capture the effects of this inlet jet mixing in order to accurately predict system performance. This paper presents a computational study of the heat transfer and fluid flow in a thermal storage tank of a solar domestic hot water system with a vertical inlet under negative buoyant plume conditions. The effects of parameters such as the fluid inlet velocity and temperature as well as inlet pipe diameter on the thermal mixing were considered. The work culminated in the development of a one-dimensional empirical model capable of predicting the transient axial temperature distribution inside the thermal storage tank. Predictions from the new model were in good agreement with both experimental data and detailed computational fluid dynamics predictions.     

Conditioning of utilizable energy by a thermostatic control of the thermosyphonic flow in solar systems

A considerable increase in the available useful energy from a thermosyphonic solar water heater which is equipped with a thermostatically flow control device is demonstrated. Such a device may reduce the system efficiency, nevertheless, it may increase the output of useful energy. The term “load utilizability” is defined and used to analyse such systems. A simulation procedure is used to evaluate the dependence of the load utilizability as function of load temperature and time. The high temperature yield of such systems facilitates their use beyond the limitation of domestic hot water supply such as industrial process and space heating. A potential reduction of the price of such systems is obtained by operating them at higher temperatures and therefore smaller water tank storage capacity. Such operational strategy increases the operational flexibility without significant loss of overall thermal efficiency.     

Thermal performance analysis of a direct-expansion solar-assisted heat pump water heater

A direct-expansion solar-assisted heat pump water heater (DX-SAHPWH) is described, which can supply hot water for domestic use during the whole year. The system mainly employs a bare flat-plate collector/evaporator with a surface area of 4.2 m², an electrical rotary-type hermetic compressor, a hot water tank with the volume of 150 L and a thermostatic expansion valve. R-22 is used as working fluid in the system. A simulation model based on lumped and distributed parameter approach is developed to predict the thermal performance of the system. Given the structure parameters, meteorological parameters, time step and final water temperature, the numerical model can output operational parameters, such as heat capacity, system COP and collector efficiency. Comparisons between the simulation results and the experimental measurements show that the model is able to give satisfactory predictions. The effect of various parameters, including solar radiation, ambient temperature, wind speed and compressor speed, has been analyzed on the thermal performance of the system.     

On the climatic optimization of the tilt and azimuth of flat-plate solar collectors

A numerical climatic model for computing total solar irradiance on the surface of a flat-plate collector, positioned at any tilt and azimuth, is described. Owing to a small time-step (one hour), and a quasi-realistic characterization of a collector's environment, the algorithm is able to produce credible estimates of both the climatically “optimal” position and the amount of energy lost to a collector when it is non-optimally positioned. Exemplary computations for Sterling, Virginia and Sunnyvale, California are presented and they suggest that the non-optimal positioning of a collector, e.g. as a simple function of latitude and a few highly summarized climatic-environmental variables, will not, in many cases, produce significant losses of available solar irradiance. In other situations, however, where a collector's horizon is significantly obstructed and/or the climatic environment of the area creates large diurnal or seasonal asymmetries in available irradiance, non-optimal positioning may cause sizeable energy losses. It is also apparent that even moderately sized horizonal obstructions, which are “seen” by a collector, can substantially reduce the amount of available irradiance, relative to an unobstructed horizon.     

Heat pump house heating system with phase change heat storage in the place of auxiliary heating

A house heating system consisting of an “energy roof” (unglazed collector), a heat pump, and medium-term phase change heat storage in the place of auxiliary heating was studied using a simple computer model. The least required storage volume is calculated and discussed as a function of the significant parameters. The choice of water for the storage medium is justified by showing the closeness of its phase change temperature to the economic optimum. The system appears technically feasible and permits minimum fossil fuel consumption together with the associated environmental benefits. Its economic position will depend on future price developments.     

Hybrid photovoltaic and thermal solar-collector designed for natural circulation of water

The electricity conversion-efficiency of a solar cell for commercial application is about 6–15%. More than 85% of the incoming solar energy is either reflected or absorbed as heat energy. Consequently, the working temperature of the solar cells increases considerably after prolonged operations and the cell’s efficiency drops significantly. The hybrid photovoltaic and thermal (PVT) collector technology using water as the coolant has been seen as a solution for improving the energy performance. Through good thermal-contact between the thermal absorber and the PV module, both the electrical efficiency and the thermal efficiency can be raised. Fin performance of the heat exchanger is one crucial factor in achieving a high overall energy yield. In this paper, the design developments of the PVT collectors are briefly reviewed. Our observation is that very few studies have been done on the PVT system adopting a flat-box absorber design. Accordingly, an aluminum-alloy flat-box type hybrid solar collector functioned as a thermosyphon system was constructed. While the system efficiencies did vary with the operating conditions, the test results indicated that the daily thermal efficiency could reach around 40% when the initial water-temperature in the system is the same as the daily mean ambient temperature.     

Life cycle assessment of a solar thermal collector: sensitivity analysis, energy and environmental balances

Starting from the results of a life cycle assessment of solar thermal collector for sanitary warm water, an energy balance between the employed energy during the collector life cycle and the energy saved thanks to the collector use has been investigated. A sensitivity analysis for estimating the effects of the chosen methods and data on the outcome of the study was carried out. Uncertainties due to the eco-profile of input materials and the initial assumptions have been analysed.Since the study is concerned with a renewable energy system, attention has been focused on the energy indexes and in particular the “global energy consumption”. Following the principles of Kyoto Protocol, the variations of CO₂ emissions have also been studied.