Performance of a “black” liquid flat-plate solar collector

A cost-effect, “black” liquid, flat-plate solar collector has been designed, and prototypes have been built and tested. In these collectors a highly absorbent “black” liquid flows in transparent channels and directly absorbs solar energy. The liquid is the hottest substance in the collector, and no metals are required anywhere in the design. The collector differs in the following ways from conventional flat-plate collectors:

1. 1. Solar radiation is absorbed directly by the black liquid without the need to heat any other structures within the collector.

2. 2. Lower heat losses are possible since energy is absorbed directly by the working fluid, and the flow pattern can be arranged so that the hottest spot is in the center of the collector away from all edges. As the fluid moves progressively inward toward the exit, which is located at the center of the collector, it will pick up some of the heat loss along the radial direction.

3. 3. Lower cost may be possible since no metal is required in construction and only glass and/or plastic need be used in addition to the insulation and frame. The absence of metal should eliminate all corrosion problems.

4. 4. New avenues of research are opened up by the use of black liquids: an entirely new class of materials are available which may aid in finding inexpensive, durable absorbers.

5. 5. New configurational arrangements are possible with the absence of metal absorbers.

Experimental performance data for the black liquid collector is presented which compares favorably with other conventional flat-plate collectors.     

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).     

Simulation of solar air heating at constant temperature

Solar space heating with warm air in typical air collectors and rock bed storage systems involves constant air flow rates and varying the temperature of supply to rooms and to storage. This practice results in undesirable fluctuations in comfort levels in the living space, excessive storage size, useful but inaccessible heat in storage, and unnecessarily high energy consumption for air circulation and auxiliary heat. These drawbacks can be avoided by use of a practical controller and variable speed fan to provide heated air from the collector at constant temperature and a continually varying flow rate. Collector manufacturer's data, confirmed by seasonal tests on a solar air heating system in Solar House II at Colorado State University, have been used in simulations at constant hot air supply temperatures of 40°, 50°, and 60°C, and at one typical constant flow rate of 49 kg/h per m² through a 50 m² collector and rock bed storage unit, providing approximately half the seasonal heating requirements of a residential building. Auxiliary heat requirements and fan power use in the 40°C and 50°C constant temperature operations were significantly reduced from the levels prevailing under constant flow conditions. Collection efficiency and solar heat supply at constant flow were slightly higher than values at the 60°C constant temperature level.     

Intergration of evacuated tubular solar collectors with lithium bromide absorption cooling systems

By surrounding the absorber-heat exchanger component of a solar collector with a glass-enclosed evacuated space and by providing the absorber with a selective surface, solar collectors can operate at efficiencies exceeding 50 per cent under conditions of ΔT/H_T = 75°C m²/kW (ΔT = collector fluid inlet temperature minus ambient temperature, H_T = incident solar radiation on a tilted surface). The high performance of these evacuated tubular collectors thus provides the required high temperature inputs (70–88°C) of lithium bromide absorption cooling units, while maintaining high collector efficiency. This paper deals with the performance and analysis of two types of evacuated tubular solar collectors intergrated with the two distinct solar heating and cooling systems installed on CSU Solar Houses I and III.     

Thermal modeling of passive and active solar stills for different depths of water by using the concept of solar fraction

This communication presents the thermal analysis of passive and active solar distillation system by using the concept of solar fraction inside the solar still with the help of AUTOCAD 2000 for given solar azimuth and altitude angle and latitude, longitude of the place. Experiments have been conducted for 24 h (9 am to 8 am) for New Delhi climatic conditions (latitude 28°35′N, longitude 77°12′E) during the months of November and December for different water depths in the basin (0.05, 0.1 and 0.15 m) for passive as well as active solar distillation system. Analytical expressions for water and glass cover temperatures and yield have been derived in terms of design and climatic parameters. It is observed that

(i) the solar fraction plays a very important role at lower values of solar altitude angle;

(ii) the internal convective heat transfer coefficient decreases with the increase of water depth in the basin due to decrease in water temperature;

(iii) there is a fair agreement between the experimental observation and theoretical prediction during daytime as compared to that during the night.

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°.     

Applied research concerning the direct steam generation in parabolic troughs

With levelized electricity costs (LEC) of 10–12 USCts/kWh the well-known SEGS (Solar Electric Generating Systems) plants in California are presently the most successful solar technology for electricity generation [Price and Cable (2001) Proc. ASME Int. Solar Energy Conf. Forum 2001]. The SEGS plants apply a two-circuit system, consisting of the collector circuit and the Rankine cycle of the power block. These two-circuits are connected via a heat exchanger. In the case of the Direct Steam Generation (DSG) in the collector field [Zarza et al. (2001) Proc. Solar Forum 2001, Washington], the two-circuit system turns into a single-circuit system, where the collector field is directly coupled to the power block. This renders a lower investment and higher process temperatures resulting in a higher system efficiency. Due to the lower investment and the higher efficiency a reduction of the LEC of 10% is expected when the DSG process is combined with improved components of the solar collectors [Zarza (2002) DISS Phase II Final Report, EU Contract No. JOR3-CT98-0277]. Within the European DISS (Direct Solar Steam) project the feasibility of the direct steam generation has been proven in more than 3700 operation hours. Steam conditions of 100 bar and 400 °C have been demonstrated. This paper presents the main scientific results of the DISS project that aims at the investigation and demonstration of the DSG process in parabolic troughs under real solar conditions.     

Preliminary results of a V-groove back-pass solar collector

The study conducted by the Solar Energy Research Group, Universiti Kebangsaan Malaysia shows that solar energy has a great potential to be used in drying processes. A prototype solar drying system was designed and tested in the campus (Othman et. al, 1994). The study shows that the system can be upgraded by improving the solar collector. This paper presents the preliminary result on the performance of the V-groove back-pass solar collector modified from V-groove double flow solar collector designed earlier (Othman et. al, 1992). The results shows that the collector maintained the output temperature eventhough there are changes of solar radiation intensity. At 0.65 ms⁻¹ air flow rate, the output temperature of the collector can manage to maintain 40°C in the range of solar intensity of 800 Wm⁻² to 300 Wm⁻² within 15 minutes time interval, and at 0.45 ms⁻¹, the output temperature is 43°C in the range of solar intensity of 800 Wm⁻² to 600 Wm⁻². The min daily efficiency of 20% is recorded at 450 Wm⁻² of min daily solar radiation at 0.65 ms⁻¹ flow rate. At 0.45 ms⁻¹ flow rate with the equal amount of daily min solar radiation the min daily efficiency is 13%. The result is expected to be better if the study is conducted in May - June when the weather is better.     

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.     

Simulation and model validation of a horizontal shallow basin solar concentrator

Salt removal from drainage water is becoming increasingly important for sustainable irrigated arid land agriculture, where inadequate drainage infrastructure exists. Solar evaporation and concentration systems are currently in development in California for this purpose. The thermal behavior and evaporation rates of a horizontal shallow basin solar concentrator were modeled for design purposes and investigated experimentally in order to validate the model. Three different evaporation rate models were evaluated and compared. Measured and predicted peak brine temperatures differed by as much as 5 °C when using prescribed literature coefficients without calibration. Model prediction was improved by calibration so that peak brine temperature deviated less than 3 °C when tested against independent data sets.Minimum root mean square error was used to calibrate the mass transfer coefficient and absorptance of the collector surface for solar radiation, which are the main factors affecting the heat transfer associated with the solar concentrator. Calibrated collector surface absorptance for solar radiation declined while mass transfer coefficients were increased from reported literature values. Under calibration, the absorptance of the collector surface was adjusted from 0.8 to 0.61, and mass transfer coefficients estimated by Newell et al. [Newell, T.A., Smith, M.K., Cowie, R.G., Upper, J.M., Cler, C.L., 1994. Characteristics of a solar pond brine reconcentration system. Journal of Solar Energy Engineering 116 (2), 69–73] from 1.36 × 10⁻⁶(1.9 + 1.065V) to 1.70 × 10⁻⁶(1.84 + 1.0V) kg m⁻² s⁻¹ mm Hg⁻¹, by Manganaro and Schwartz [Manganaro, J.L., Schwartz, J.C., 1985. Simulation of an evaporative solar salt pond. Industrial & Engineering Chemistry Process Design and Development 24, 1245–1251] from 0.0208(1 + 0.224V) to 0.0233(1 + 0.214V) kg m⁻² h⁻¹ mm Hg⁻¹, and by Alagao et al. [Alagao, F.B., Akbarzadeh, A., Johnson, P.W., 1994. The design, construction, and initial operation of a closed-cycle, salt-gradient solar pond. Solar Energy 53 (4), 343–351] from 2.8 + 3.0V to 3.0 + 3.33V W m⁻² °C⁻¹. The calibrated models were tested using an independent data set. Maximum deviation between measured and predicted brine temperatures differed by less than 3 °C. The measured and predicted peak evaporation rates were between 1.2 and 1.4 kg m⁻² h⁻¹.The calibrated Newell model was used to predict the monthly productivity and daily maximum evaporation rates at Five Points, California for the year 2004. The productivity from April to September and from March to October was 80.7% and 94.3% of the total annual productivity, respectively.     

Heat gain reduction by means of thermoelectric roof solar collector

This paper presents a numerical investigation on attic heat gain reduction by using thermoelectric modules integrated in a conventional roof solar collector (RSC). This system, called thermoelectric roof solar collector (TE-RSC), is composed of a transparent glass, air gap, a copper plate, thermoelectric modules (TE) and rectangular fin heat sink. Due to the incident solar radiation, a temperature difference is created between the hot and cold sides of TE modules that generates a direct current. This current is used to drive a ventilating fan for cooling the TE-RSC and enhancing attic ventilation that reduces ceiling heat gain. The system performance was simulated using TRNSYS program with new TE and DC fan components developed by our team and compared to a common house.Simulation results using real house configuration showed that a TE-RSC unit of 0.0525 m² surface area can generate about 9 W under 972 W/m² global solar radiation and 35 °C ambient temperature. The induced air change varied between 20 and 40 and the corresponding ceiling heat transfer rate reduction is about 3–5 W/m². The annual electrical energy saving was about 362 kWh. Finally, economical calculations indicated that the payback period of the TE-RSC is 4.36 years and the internal rate of return is 22.05%.     

First solar radiation atlas for the Arab world

In the year 1998, the Arab League Educational, Cultural and Scientific Organization (ALESCO), Directorate of Science and Scientific Research, Tunis, had launched the “Solar Radiation Atlas for the Arab World”. This atlas contains three sets of maps (using Mercator projection) for monthly means, where each stands for one month. These are sunshine duration, global solar radiation and diffuse solar radiation. The atlas contains data for nearly 280 stations from 19 Arab states which cover latitudes from 0° (tropic) to 37°N and longitudes 19°E to nearly 60°E with different elevations from the sea level. It also contains useful tables of the monthly recorded means of the direct, diffuse and global solar radiation as well as the sunshine duration for 16 Arab states including 207 cities.The maximum recorded annual mean (10 years) of the global solar radiation in the Arab world was 6.7 kW h/m²/day in Nouakchott (latitude 20°56′N, longitude 17°02′E), Mauritania, and 6.6 kW h/m²/day in Tamenraset (latitude 36°11′N and longitude 5°31′E), Algeria, while the lowest recorded annual mean global solar radiation was 4.1 kW h/m²/day in Mosul (latitude 43°N and longitude 36°E), Iraq. Furthermore, the maximum recorded annual mean sunshine duration in the Arab world was 10.7 h in Aswan (latitude 23°58′N, longitude 32°47′E), Egypt, and the lowest was 7.5 h in Tunis (latitude 36°50′N, longitude 10°14′E), Tunisia.     

Long-term (18 years) performance of a residential solar heating system

The long-term performance of a residential solar heating system has been determined for a system which has been operating continuously since 1957 with no maintenance. This residential solar heating system is the Colorado Solar House located in Denver, Colorado, designed and operated by George O. G. Löf.The performance of this system was determined during the 1959–1960 heating season, and the results were publised. The performance of this system was redetermined during the 1974–1975 heating season so that changes in performance occurring over a period of 15 yr could be determined.The collector is an Overlapped-Glass Plate Solar-Air Heater. The system is completely automatic with provision for water heating in addition to space heating. Solar heat is stored in a rock bed of primarily granitic rock approximately 1.3–2.5 cm in diameter.The ratio of useful collected solar heat divided by the total solar radiation on the collector dropped to 71.8 per cent of its original value in 15 yr. For both seasons, the useful collected solar heat was correlated with the ratio of degree days per month divided by the total solar radiation on the collector. For the same value of this ratio, less useful collected solar heat was delivered during the latter season.Additional work that will be published at a later date includes the detailed performance of the hot water heating system.     

An autonomous solar ammonia-water refrigeration system

Solar refrigeration is especially attractive in isolated regions. The Danube Delta needs ice for fish preservation. This paper describes an intermittent single-stage H₂O---NH₃ solar absorption system of 46 MJ/cycle. Solar collectors heat the generator. Installation details and experimental results are presented. The system coefficient of performance (COP)_system varies between 0.152 and 0.09 in the period of May–September. Solar radiation availability and the theoretical (COP), also applicable to the Trombe-Foex system, are assessed. Reference is made to evacuated solar collectors with selective surfaces. Actual (COP)_system values of 0.25–0.30 can be achieved at generation and condensation temperatures of 80°C and 24.3°C respectively. For bigger capacities of 450–675 MJ/day, the pay-off period is estimated to be 6 and 4 years respectively and the life-time to 15–18 years.     

The influence of construction parameters on the performance of a low cost flat-plate collector

A simple flat-plate solar water heater which can be used to heat water for domestic needs, especially drinking water for villagers, has been constructed using locally available materials such as wood, saw-dust, sponge and palm-tree fibre. With palm-tree fibre as the best local insulator, the collector was used to heat water trapped in it to about 98°C. Using a natural circulation (thermosiphon) method, water in a tank was heated to about 42°C. An experimental approach has also been used to analyse the effects of construction parameters on the collector performance. Results show that at ambient temperatures, T_a, greater than 23°C (i.e. T_a > 23°C) the use of three or more transparent glass covers improves the collector efficiency because they make the collector less sensitive to wind speed and to slight changes in the ambient temperature of the surroundings. Efficiencies of 9.1%, 9.5% and 9.8% were recorded for one, two and three glass covers respectively when palm-tree fibre was used. It was also noted that a tilt angle of about 7°, which corresponds fairly well with the latitude of the experimental site of 7° for Ile-Ife gave optimum collector output temperature.     

Ditigal simulation of indoor temperatures of buildings with roof ponds

A theoretical model is presented to predict the thermal performance of a building with roof ponds. Equations have been derived for the estimation of steady periodic heat fluxes through the roof slab and the outer walls. Energy storage and release by the partition walls and the floor has been considered. The other cooling loads have been estimated using the methods recommended in the ASHRAE Guide and Data Book. Hourly indoor temperatures are obtained by the numerical solution of the energy balance equation for the building. The algorithm that has been developed for digital simulation of the indoor temperatures is presented. The effectiveness of different kinds of roof-pond systems, i.e. shaded ponds, “Sky-therm”, etc. for passive coolings have been examined. The studies indicate that the indoor temperatures of a building located in Delhi can be maintained below 30°C in summer while the maximum dry-bulb temperatures are above 40°C.     

A new transparently insulated, bifacially irradiated solar flat-plate collector

A new type of transparently insulated flat-plate collector was developed. It reaches higher efficiencies at low irradiation values or high operating temperatures than any other collector type known. Both sides of its absorber are covered with transparent insulation material and both sides are irradiated. Thus, the heat losses of the collector related to the total absorber area are distinctly reduced. An optical efficiency of η₀ = 0.72 and a temperature dependent U-value of U(ΔT) = (0.95 + 0.0076 ΔTK⁻¹) W m⁻² K⁻¹ were measured with an outdoor test facility. The bifacial-absorber collector is considered to be the best option for the DHW system of the energetically self-sufficient solar house in Freiburg because of its outstanding winter performance.     

Is there a need for a rock bed store? Simulation and optimization of solar air heating systems for offices with large thermal capacity walls

A solar air heating system is designed for a floor of 120 m² offices, with large thermal capacity walls, in Israel. A constant air volume system is chosen for its operational simplicity. Representative winter hourly weather data are used to calculate the heating load. The building behavior is modeled in detail with dynamic wall and room temperatures which are linked to the heat input. The heat losses are found to be primarily (70–75%) due to storage in the walls for two different values of wall heat capacity and for two design temperatures (19° and 20°C). The paper deals with the operational details, seasonal performance and economics of the system. Multivariate optimization is carried out using the Simplex method. Optimum collector area, store volume and air flow rate of 30 m², 2–3 m³ and 0.5 kg s⁻¹, respectively, are not affected by economic predictions. A comparison of this system with one which omits the rock bed store and uses only the building material as storage is also made. Results show that for the higher design temperaturre of 20°C, the rock bed store improves system performance, but the same solar fraction can be achieved by increasing the collector area from 30 to 50 m² in the system without active storage. For the lower design temperature of 19°C the improvement in performance made by the addition of the rock bed store is small, and can be obtained by increasing the collector area from 30 to 40 m², obviating the need for the store system. In buildings with a high heat capacity, operated during daytime only, the no-active-store system is recommended for its ease of operation and suitability for retrofitting.     

HIGH EFFICIENCY EVACUATED FLAT-PLATE SOLAR COLLECTOR FOR PROCESS STEAM PRODUCTION

A flat-plate solar collector for process steam production was developed. The operating temperatures are in the range between 100 and 150°C. The boiling collector can be used for process heat supply in the industry and for solar cooling applications as well. It operates as a system with controlled influx of liquid. Instabilities of the two-phase flow in the internal evaporator have been successfully suppressed and the design of the system has been investigated. We constructed a prototype collector based on a commercially available evacuated flat-plate collector. To realise high thermal efficiencies at temperatures up to 150°C, the thermal losses of the absorber have been drastically reduced using an ultra low emissive selective absorber, a low pressure krypton filling (50 hPa) in the collector casing, and a highly reflecting aluminium foil between absorber and rear side. The prototype collector was dynamically tested at our outdoor test facility and showed very high efficiencies of more than 60% at 100°C steam temperature and of 45% at 150°C steam temperature (T_amb=15°C). The operation behaviour of the prototype was always stable and the steam mass quality showed excellent values of nearly 100%.     

A stationary evacuated collector with integrated concentrator

A comprehensive set of experimental tests and detailed optical and thermal models are presented for a newly developed solar thermal collector. The new collector has an optical efficiency of 65 per cent and achieves thermal efficiencies of better than 50 per cent at fluid temperatures of 200°C without tracking the sun. The simultaneous features of high temperature operation and a fully stationary mount are made possible by combining vacuum insulation, spectrally selective coatings, and nonimaging concentration in a novel way. These 3 design elements are “integrated” together in a self contained unit by shaping the outer glass envelope of a conventional evacuated tube into the profile of a nonimaging CRC-type concentrator. This permits the use of a first surface mirror and eliminates the need for a second cover glazing. The new collector has been given the name “Integrated Stationary Evacuated Concentrator”, or ISEC collector. Not only is the peak thermal efficiency of the ISEC comparable to that of commercial tracking parabolic troughs, but projections of the average yearly energy delivery also show competitive performance with a net gain for temperatures below 200°C. In addition, the ISEC is less subject to exposure induced degradation and could be mass produced with assembly methods similar to those used with fluorescent lamps. Since no tracking or tilt adjustments are ever required and because its sensitive optical surfaces are protected from the environment, the ISEC collector provides a simple, easily maintained solar thermal collector for the range 100–300°C which is suitable for most climates and atmospheric conditions. Potential applications include space heating, air conditioning, and industrial process heat.