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.     

A study of Canadian diffuse and total solar radiation data—II Monthly average hourly horizontal radiation

Optimum collector slope for a liquid base active solar heating system employing flat-plate collectors was investigated. The optimum collector slope was studied as a function of (a) collector area, (b) yearly total heating load and (c) the ratio of space heating load to service hot water load. Collectors facing equator only were considered. Such a system was studied in four different Canadian locations having widely different climates. Under the above conditions, optimum collector slope varied with the amount of collector area employed. The optimum collector slope was invariant with the yearly total load itself, or the space heating to hot water load ratio. Contrary to the widely held belief, for the four locations investigated, the optimum collector slope varied from lat. − 10° to lat. + 15°; depending upon f_y, the fraction of load supplied by the solar system. When f_y is in 10–20 per cent range, optimum collector slope is lat. − 10° and increases almost linearly to lat.+ 15° at f_y in 80 per cent range. Consequently, when the fraction of load by the solar system is low, a flat roof may be profitably employed. On the other hand, when the fraction by the solar system is high, a south facing (for northern hemisphere) vertical wall may be profitably employed.     

Exergy analysis of integrated photovoltaic thermal solar water heater under constant flow rate and constant collection temperature modes

In this communication, an analytical expression for the water temperature of an integrated photovoltaic thermal solar (IPVTS) water heater under constant flow rate hot water withdrawal has been obtained. Analysis is based on basic energy balance for hybrid flat plate collector and storage tank, respectively, in the terms of design and climatic parameters. Further, an analysis has also been extended for hot water withdrawal at constant collection temperature. Numerical computations have been carried out for the design and climatic parameters of the system used by Huang et al. [Huang BJ, Lin TH, Hung WC, Sun FS. Performance evaluation of solar photovoltaic/thermal systems. Sol Energy 2001; 70(5): 443–8]. It is observed that the daily overall thermal efficiency of IPVTS system increases with increase constant flow rate and decrease with increase of constant collection temperature. The exergy analysis of IPVTS system has also been carried out. It is further to be noted that the overall exergy and thermal efficiency of an integrated photovoltaic thermal solar system (IPVTS) is maximum at the hot water withdrawal flow rate of 0.006 kg/s. The hourly net electrical power available from the system has also been evaluated.     

Experimental study of a multi-tube ICS solar water heating system in mild climates

Since a majority of residential and industrial building hot water needs are around 50°C, an integrated solar water heater could provide a bulk source that blends collection and storage into one unit. These collectors incorporate thermal storage within the collector itself. The storage pipe surface serves as the absorber surface. Most ICS systems use only one tank, but some use a number of tanks in series. While the simplicity of ICS systems is attractive, they are generally suitable only for applications in mild climates with small thermal storage requirements. A multi-tube ICS solar hot water system with eight cylindrical water storage tanks (pipes) in horizontal (East-West) orientation was designed and outdoor tests of experimental model were performed in mild climate of north (Caspian Sea) region of Iran. Experimental results such as water temperature profiles, mean daily efficiency, water temperature stratification and thermal losses during night are presented and discussed for this model. Experimental results showed that the daily mean efficiency is comparable to other ICS systems and also acceptable thermal performance of this type of ICS system has been observed.     

Simulation analysis for the active solar heating system of a passive house

The active solar heating system consists of the following sub-systems: (1) a solar thermal collector area, (2) a water storage tank, (3) a secondary water circuit, (4) a domestic hot water (DHW) preparation system and (5) an air ventilation/heating system. An improved model for the secondary water circuit is proposed and two interconnection schemes for sub-systems (4) and (5) are analyzed. The integrated model was implemented to Pirmasens passive house (Rhineland Palatinate, Germany). Both interconnection schemes show that (almost all) the solar energy collected is not used for space heating but for domestic hot water preparation. The classical water heater operates all over the year and the classical air heater operates mainly during the nights from November to April. The yearly amount of heat required by the DHW preparation system is about 77% of the yearly total heat demand of the passive house and the classical water heater provides about 20% of the yearly heat required by the DHW preparation system. The solar fraction lies between 0.247 in January and 0.930 in August, with a yearly average of 0.597.     

Heat retaining integrated collector/storage solar water heaters

An integrated collector/storage solar water heater (ICSSWH) that can significantly reduce heat loss to ambient during non-collection periods has been developed. Two thirds of the ICS vessel is mounted within a concentrating cusp, McIntire ‘W’ modified concentrator and incorporates an inner heat retaining vessel. The remaining upper 1/3 of the vessel is situated outside the reflector cavity and is heavily insulated. Over 60% of the thermal energy stored within the total vessel, and up to 67% of that in the upper immediate draw-off region can be retained over a 16-h non-collection period. Results of an experimental analysis of this design and a comparison with a standard ICS design are presented.     

带透明蜂窝盖板和辅助反射面的整体式太阳热水器

作者对一种带透明蜂窝盖板和辅助反射面的整体式(ICS)太阳热水器进行了实验研究。该太阳热水器采用截面为三角形的水箱，水箱背面和侧面用30mm聚苯乙烯泡沫隔热，其它两个面为吸热面。底吸热面利用辅助反射面加热，而上吸热面则覆盖5cm的透明蜂窝及2mm的有机玻璃板。这种设计加大了ICS太阳热水器的吸热面积，同时也降低了吸热面向环境的热损。对实验结果的分析表明，该热水器的热效率不高，但保温性能很好。     

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.     

Hybrid photovoltaic-thermosyphon water heating system for residential application

In order to improve the energy performance of the photovoltaic system, much effort has been spent on the research and development of hybrid PVT (photovoltaic-thermal) technology using water as the coolant. The fin performance of the thermal absorber is known to be one crucial factor in achieving a high overall energy yield of the collector. Accordingly, an aluminum-alloy flat-box type PVT collector was constructed, with its fin efficiency approaching unity. Its design is primarily for natural circulation and for domestic water heating purpose. Our test results showed that a high final hot water temperature in the collector system can be achieved after a one-day exposure. A numerical model of this photovoltaic-thermosyphon collector system was also developed and the model accuracy was verified by comparison with measured data. The energy performance of the collector system was then examined first, through reduced-temperature analysis, and second, as applying in the “hot summer and cold winter” climate zone of China. The numerical results are found very encouraging, and the equipment is capable of extending the PV application potential in the domestic sector.     

Optimizing the tilt angle of solar collectors

Solar collectors need to be tilted at the correct angle to maximize the performance of the system. In this paper, the annual solar fraction of the system (the fraction of energy that is supplied by solar energy) is used as an indicator to find the optimum inclination angles for a thermosyphon solar water heater installed in northern and southern parts of Jordan. Calculations are carried out using the powerful computer program TRNSYS (Transient System Simulation). The system is assumed to operate with a daily hot water load of 150 l at 55°C flowing during the day according to the widely used Rand consumption profile. The results show that the optimum inclination angle for the maximum solar fraction is about φ+(0→10°) for the northern region (represented by Amman) and about φ+(0→20°) for the southern region (represented by the town of Aqaba). These values are greater than those for maximum solar radiation (which is commonly used as an indicator) at the top of the collector by about 5 to 8°.     

一种新型的与建筑一体化太阳能双效集热器系统的实验研究

提出了一种新型的与建筑一体化太阳能双效集热器系统,该系统有两种工作模式:被动采暖工作模式和集热水工作模式。由系统工作在被动采暖工作模式下的实验结果可以得到:系统房间内空气存在温度分层现象,测试期间上下位置最大温差为4.2℃,平均温差约为2.7℃;系统在被动采暖工作模式下工作时对房间温度的提高作用明显,实验测试结果显示,实验期间系统房间平均温度达到24.7℃,相对环境温度升高平均达到19.9℃。通过系统以自然循环方式工作在集热水工作模式下的实验测试结果,可以得到实验期间在集热水工作模式下系统的热效率为52.8%,集热器单位面积太阳得热为4.16MJ/m~2。     

A “once-through” solar water heating system

In China, solar water heater is being popularized and most existing solar water heaters are the natural circulation system. Due to some shortcomings of the natural circulation system, a “once-through” system is proposed. In a once-through system, the storage tank can be placed below the collector, therefore, the load on the roof will be cut down significantly. This system also has the advantages of no mixing of hot and cold water, no reverse flow and being able to provide hot water earlier, etc. Both theoretical and experimental investigations have been conducted to compare the collector efficiences between the once-through and natural circulation systems and they coincide with each other very well. The once-through system performs worse in the morning but better in the afternoon than the natural circulation system and the difference of daily collector efficiency between these two systems is negligible.     

Optimum collector slope for residential heating in adverse climates

Optimum collector slope for a liquid base active solar heating system employing flat-plate collectors was investigated. The optimum collector slope was studied as a function of (a) collector area, (b) yearly total heating load and (c) the ratio of space heating load to service hot water load. Collectors facing equator only were considered. Such a system was studied in four different Canadian locations having widely different climates. Under the above conditions, optimum collector slope varied with the amount of collector area employed. The optimum collector slope was invariant with the yearly total load itself, or the space heating to hot water load ratio. Contrary to the widely held belief, for the four locations investigated, the optimum collector slope varied from lat. ? 10° to lat. + 15°; depending upon f_y, the fraction of load supplied by the solar system. When f_y is in 10–20 per cent range, optimum collector slope is lat. ? 10° and increases almost linearly to lat.+ 15° at f_y in 80 per cent range. Consequently, when the fraction of load by the solar system is low, a flat roof may be profitably employed. On the other hand, when the fraction by the solar system is high, a south facing (for northern hemisphere) vertical wall may be profitably employed.     

Numerical and experimental study of an integrated solar collector with CPC reflectors

The aim of this work is to develop a numerical code able to predict the thermal behavior of a double tank integrated collector storage system (ICS) with compound parabolic concentrator (CPC). The developed numerical model is based on the detailed analysis of the different forms of heat transfers occurring in the ICS system. The balance equations of each element of the system have been established and solved by means of a transient algorithm. A prototype of an ICS device was constructed and experimentally tested outdoors in order to observe the variation of water temperature in the storage tanks. The experimental results are presented and the validity of the model is examined by comparison of the theoretical results with experiments which demonstrates a good agreement. The numerical model is then used to perform theoretical study on the present ICS solar heater. The simulation results of the variation of the thermal efficiency are presented. The results of the yearly parametric study of the effect of the concentrators reflectivity, the absorber emissivity and the use of double glazing on the thermal performance of the ICS system are also presented and discussed. The developed numerical tool within this work can be considered as important for the study of double tanked ICS solar water heater regarding its transient thermal behavior.     

Experimental investigation and simulation analysis of the thermal performance of a balcony wall integrated solar water heating unit

A balcony wall type solar water heater system was designed and constructed in a high-rise building. The U-type evacuated glass tube solar collector is fixed vertically on the balcony wall. The water, heated in the solar collector, flows through the exchanger coil in the water tank and then flows back to the solar collector. With regard to the hot water supply system, the cold water, heated by the heat exchanger, is sent to the point of use. Considering storeys and water consumption pattern, four apartments are selected for testing. Meanwhile, the theoretical analysis with TRNSYS was presented. According to the experimental results, mean daily collector efficiency is about 40%. Solar fraction is high in summer and autumn for the relative high radiation and high ambient temperature. Under given conditions, the annual energy extracted from tank is 2805.3 MJ/m², and the annual solar fraction is 40.5%. When the tank volume-to-collector area ratio is decreased to 37.5 L/m², the solar fraction can be increased to 50%. The results show that the family to use water all day round gets higher solar fraction than the family using hot water mostly in the morning and night.     

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).     

A REVIEW OF LARGE-SCALE SOLAR HEATING SYSTEMS IN EUROPE

Large-scale solar applications benefit from the effect of scale. Compared to small solar domestic hot water (DHW) systems for single-family houses, the solar heat cost can be cut at least in third. The most interesting projects for replacing fossil fuels and the reduction of CO₂-emissions are solar systems with seasonal storage in combination with gas or biomass boilers. In the framework of the EU–APAS project “Large-scale Solar Heating Systems”, thirteen existing plants in six European countries have been evaluated. The yearly solar gains of the systems are between 300 and 550 kWh per m² collector area. The investment cost of solar plants with short-term storage varies from 300 up to 600 ECU per m². Systems with seasonal storage show investment costs twice as high. Results of studies concerning the market potential for solar heating plants, taking new collector concepts and industrial production into account, are presented. Site specific studies and predesign of large-scale solar heating plants in six European countries for housing developments show a 50% cost reduction compared to existing projects. The cost–benefit-ratio for the planned systems with long-term storage is between 0.7 and 1.5 ECU per kWh per year.     

Thermal advantages for solar heating systems with a glass cover with antireflection surfaces

Investigations elucidate how a glass cover with antireflection surfaces can improve the efficiency of a solar collector and the thermal performance of solar heating systems. The transmittances for two glass covers for a flat-plate solar collector were measured for different incidence angles. The two glasses are identical, except for the fact that one of them is equipped with antireflection surfaces by the company SunArc A/S. The transmittance was increased by 5–9%-points due to the antireflection surfaces. The increase depends on the incidence angle. The efficiency at incidence angles of 0° and the incidence angle modifier were measured for a flat-plate solar collector with the two cover plates. The collector efficiency was increased by 4–6%-points due to the antireflection surfaces, depending on the incidence angle. The thermal advantage with using a glass cover with antireflection surfaces was determined for different solar heating systems. Three systems were investigated: solar domestic hot water systems, solar heating systems for combined space heating demand and domestic hot water supply, and large solar heating plants. The yearly thermal performance of the systems was calculated by detailed simulation models with collectors with a normal glass cover and with a glass cover with antireflection surfaces. The calculations were carried out for different solar fractions and temperature levels of the solar heating systems. These parameters influence greatly the thermal performance associated with the antireflection surfaces.     

Collector cum storage solar water heaters with and without transparent insulation material

The integrated collector–storage solar water heaters are less expensive and can offer the best alternative for domestic applications particularly to small families to meet hot water requirements. The top heat losses of such solar water heaters are quite high during the night and the temperature of stored hot water is considerably reduced unless covered with extra insulating cover in the evening which is a cumbersome job. The transparent insulation material widely used in Europe for space heating can also minimize top heat losses, if used in such solar water heaters. For this purpose, two units of solar collector cum storage water heaters have been designed to study the relative effect of TI for retaining solar heated hot water for a night duration. Both units were identical in all respects except one of them was covered with TIM. The theoretical exercise was carried out to evaluate design parameters of ISC which revealed total heat loss factor (U_L) 1.03 W/m² K with TIM glazed against 7.06 W/m² K with glass glazed. The TIM glazed has been found to be quite effective as compared to glass glazed SWH and yielded hot water at higher temperature by 8.5 to 9.5°C the next morning. The storage efficiency of such solar water heaters has been found to be 39.8% with TIM glazed as compared to 15.1% without TIM. The TIM glazing means not having to cover the ISC solar water heater with a separate insulator cover in the evening and thus makes its operation much simpler.     

Solar houses/heating and cooling progress report

Performance data on seven solar homes are given. Solar Homes No. 1, 2, 3, and 4 are near Washington, D.C., 39° north latitude, where about half of the winter days are cloudy and temperatures drop far below freezing, sometimes to 0°F. These houses are described in the book Solar Houses and Solar House Models by Harry E. Thomason, published by Edmund Scientific Company, Barrington, New Jersey, 08007. Edmund Scientific Co. also publishes Solar House Plans, for building a house similar to Solar House No. 1, with improvements.

1. Solar House No. 1Solar House No. 1 has been in continuous operation for thirteen years. In its first year, solar heat supplied about 95 per cent of the heat requirements for home temperatures at 70°F, plus or minus 2°F. After 5 yr of operation, the heat collector was rebuilt. Longer-lived materials were used although efficiency was lowered somewhat. Also changes were made in the air conditioning system.

2. Solar House No. 2A number of changes were incorporated in House No. 2, built in 1960 and 1961. Cost for the original system was lowered, but the auxiliary heat cost ran slightly higher. An aluminum reflector was installed at the bottom of the solar heat collector to reflect additional sunlight onto the collector. The air conditioning system in House No. 2 is rather satisfactory, and that type of system is now in House No. 1 as well as in House No. 2.Summertime heat leakage from the solar heat collector into the closet space behind the collector was solved in House No. 2. The closet remains cool. However, at times the temperature in the closet drops too low and a new problem has to be solved by re-introducing heat to the closet.

3. Solar House No. 3The architectural appearance of House No. 3 was improved. Low-cost glazing with a minimum of glass breakage was achieved. The heat collector was moved entirely up to the roof so that winter sunshine enters the living room and built-in swimming pool on the south side. Improved air conditioning was installed.

4. Solar House No. 4A new type of solar heat collector (with asphalt shingles) and a new type of low-cost “Pancake” heat storage were incorporated into this A-frame house.

5. Solar House No. 5House No. 5, planned for a South Carolina firm, was never built due to insufficient funds.

6. Solar House No. 6House No. 6 has been completed in Mexico City, Mexico. The house and system were not constructed as the authors recommended so the solar heating system does not provide the major part of the heat load. Although Mexico City is quite far south (19° north latitude), the temperature drops below the freezing mark at times. (On December 8, 1970, the temperature dropped to 24°F.)

7. Solar House No. 7The authors have designed and engineered a new system, their Solaris “Sunny South Model,” with three principle innovations. (1) The system uses “Pancake” under-the-floor heat storage. (2) The system utilizes a shallow roof-pond solar heat collector with a reflector to intensify solar input. (3) The system allows the warm water from the roof-pond to drain each night to the under-floor “Pancake” heat storage area where it warms the floor and living space. During the summer the roof-pond helps minimize day-night temperature extremes by absorbing excess heat during the day and liberating it at night.