Smart solar tanks for small solar domestic hot water systems

Investigation of small SDHW systems based on smart solar tanks are presented. The domestic water in a smart solar tank can be heated both by solar collectors and by means of an auxiliary energy supply system. The auxiliary energy supply system––in this study electric heating elements––heats up the hot-water tank from the top and the water volume heated by the auxiliary energy supply system is fitted to the hot-water consumption and consumption pattern. In periods with a large hot-water demand, the volume is large; in periods with a small hot-water demand, the volume is small.Two small SDHW systems, based on differently designed smart solar tanks and a traditional SDHW system were investigated by means of laboratory experiments and theoretical calculations. The investigations showed that the yearly thermal performance of SDHW systems with smart solar tanks is 5–35% higher than the thermal performance of traditional SDHW systems. Estimates indicate that the performance/cost ratio can be improved by up to 25% by using a smart solar tank instead of a traditional tank when the backup energy system is electric heating elements. Further, smart solar tanks are suitable for unknown, variable, large or small hot-water consumption and the risk of oversized solar heating systems and oversized tank volumes is reduced by using smart solar tanks. Based on the investigations it is recommended to start development of smart solar tank units with an oil-fired boiler or a natural gas burner as auxiliary energy supply system.     

Design of a solar collector for year-round climatization

Solar heating systems in buildings have increasingly been studied in the past two decades. In several applications the primary energy demand is now for both heating and cooling, and modern solar collectors should be designed to provide climatization during the whole year. Solar systems are seldom applied in Europe, and large buildings, such as office buildings and schools, continue to be built with mechanical ventilation systems.The study presented in this paper is part of a European XVII Thermie project entitled “Pilot project for photovoltaic, energetic and biohousing retrieval in a school”, whose aim was to install a photovoltaic plant and solar air collectors coupled with a sun breaker structure at a scientific high school in Umbertide, in central Italy.This paper describes the research and development activities concerning a solar air collector suited for winter heating and summer ventilation, which was installed at the high school. The collector physical and numerical modelling of heat transfer and fluid flow in winter operation is presented. The system performance has been estimated as a function of different parameters in order to provide a tool for the design process. Furthermore, the climate in the area has been simulated through the available experimental data, and the system behavior under these conditions is presented.The collectors were installed at the scientific high school in Umbertide in spring 2001. Summer ventilation cooling is under testing and an experimental test period is foreseen next winter to validate the design of the collectors and their performance.     

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 and economical analysis of a central solar heating system with underground seasonal storage in Turkey

Thermal performance and economic feasibility of two types of central solar heating system with seasonal storage under four climatically different Turkey locations are investigated. The effects of storage volume and collector area on the thermal performance and cost are studied for three load sizes. The simulation model of the system consisting of flat plate solar collectors, a heat pump, under ground storage tank and heating load based on a finite element analysis and finite element code ANSYS™ is chosen as a convenient tool. In this study, the lowest solar fraction value for Trabzon (41°N) and the highest solar fraction value for Adana (37°N) are obtained. Based on the economic analysis, the payback period of system is found to be about 25–35 years for Turkey.     

Numerical simulation and performance assessment of a low capacity solar assisted absorption heat pump coupled with a sub-floor system

A prototype low capacity (10 kW) single stage Li–Br absorption heat pump (AHP), suitable for residential and small building applications has been developed as a collaborative result between various European research institutes and industries. The primary heat source for the AHP is supplied from flat plate solar collectors and the hot/chilled water from the unit is delivered to a floor heating/cooling system. In this paper we present the simulation results and an overview of the performance assessment of the complete system. The calculations were performed for two building types (high and low thermal mass), three climatic conditions, with different types of solar collectors and hot water storage tank sizes and different control systems for the operation of the installation. The simulations were performed using the thermal simulation code TRNSYS. The estimated energy savings against a conventional cooling system using a compression type heat pump was found to be in the range of 20–27%.     

Design considerations for residential solar heating and cooling systems utilizing evacuated tube solar collectors

As solar heating systems become a commercial reality, greater efforts are now being employed to incorporate solar cooling components in order to obtain a complete solar heating and cooling system and thus take advantage of the cost-effectiveness of year-round use of the solar equipment. Because of the exceptional performance and high efficiency of evacuated tube solar collectors, these advanced collectors are receiving considerable attention for use in solar heating and cooling systems. While improved performance is readily obtained with these sophisticated solar collectors, there are also numerous difficulties and problems associated with their use in a solar system. This paper addresses many of the design considerations which must be included in any realistic solar system design. Most of the considerations presented here are based on the experience gained in the design and performance of the solar heating and cooling systems for CSU Solar Houses I-IV.     

Economic analysis of photovoltaic-powered solar domestic hot water systems in Kuwait

The present paper first presents a comparison between the photovoltaic four-parameter and five-parameter models. The comparison was performed using polycrystalline and amorphous silicon solar cells. The results showed that the four-parameter model can accurately determine the I–V characteristics of single- and polycrystalline solar cells. On the other hand, the five-parameter model can be used to accurately determine the I–V characteristics of amorphous solar cells in addition to its capability of predicting the I–V characteristics of single- and polycrystalline solar cells.Secondly, the economic feasibility of photovoltaic-powered solar water heating systems in Kuwait has been examined. The system includes photovoltaic arrays instead of solar collectors used in common solar water heating systems. The five-parameter model was used to simulate the performance of amorphous silicon modules used with the photovoltaic-powered solar water heating systems. Oil prices in the range of US$20–30 per barrel were studied. At the current prices of photovoltaic modules and a low oil price, photovoltaic-powered solar water heating systems were found to be economical. On the other hand, assuming a low oil price, the price of photovoltaic modules should be reduced to 50% of their current prices to make photovoltaic-powered solar water heating systems feasible in the Kuwaiti climate.     

Dynamic study of the thermal behaviour of solar thermochemical refrigerator: barium chloride-ammonia for ice production

The results of the theoretical thermodynamic analysis and the dynamic behaviour of the solar heating system of a thermochemical refrigerator, which operates on a heterogeneous solid–gas reaction between barium chloride and ammonia, are presented in this work. The thermodynamic analysis of the barium chloride–ammonia system shows that after energy and mass balance, the global efficiency coefficient (COP) varies very little. The theoretical relative low temperatures of dissociation in this system which are between 50°C and 60°C need simple heating systems such as flat plate collectors are needed, with an advantage over traditional liquid/vapour absorption systems. A simulation of the annual dynamic behaviour of the solar heating system for the operation of a solid-gas reactor is also presented. For an ice production specific cooling load, calculations are made of the different solar fractions of different areas of solar caption as well as the monthly variations of the efficiencies of the refrigeration systems.     

Thermal performances of the active and passive water heating systems based on annual operation

In this paper, the year round thermal performance evaluation of active and passive solar water heating systems for rural/urban area have been studied. Two types experiments are the normalized draw-off temperature profile and the normalized draw-off mixing profile conducted in the months of April 2003–March 2004. Effect of morning, mid-afternoon, evening draw-off and effect of water mixing in the morning and evening are also studied during the experiments. Thermal efficiency for the system and collection efficiency for the collectors were calculated during the experiments. The values of F_R · (τα) of the collectors used in active and passive water heating systems were found 0.69 and 0.61 respectively. Ethylene glycol has a 50/50 glycol-to-water ratio used as a heat transfer fluid (HTF) in the systems due to the cold climate in South Korea.     

Mathematical modeling and thermal performance analysis of unglazed transpired solar collectors

Unglazed transpired collectors or UTC (also known as perforated collectors) are a relatively new development in solar collector technology, introduced in the early nineties for ventilation air heating. These collectors are used in several large buildings in Canada, USA and Europe, effecting considerable savings in energy and heating costs. Transpired collectors are a potential replacement for glazed flat plate collectors. This paper presents the details of a mathematical model for UTC using heat transfer expressions for the collector components, and empirical relations for estimating the various heat transfer coefficients. It predicts the thermal performance of unglazed transpired solar collectors over a wide range of design and operating conditions. Results of the model were analysed to predict the effects of key parameters on the performance of a UTC for a delivery air temperature of 45–55 °C for drying applications. The parametric studies were carried out by varying the porosity, airflow rate, solar radiation, and solar absorptivity/thermal emissivity, and finding their influence on collector efficiency, heat exchange effectiveness, air temperature rise and useful heat delivered. Results indicate promising thermal performance of UTC in this temperature band, offering itself as an attractive alternate to glazed solar collectors for drying of food products.The results of the model have been used to develop nomograms, which can be a valuable tool for a collector designer in optimising the design and thermal performance of UTC. It also enables the prediction of the absolute thermal performance of a UTC under a given set of conditions.     

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.     

Computational evaluation of solar heating systems using concrete solar collectors

This paper uses the F-chart technique to evaluate three types of solar heating systems, namely; space solar heating and domestic hot water system (SHDHW), domestic hot water system (DHW) and solar swimming pool heating system (SPHS), using three types of concrete solar collectors, models A, B, and C, and one conventional metallic solar collector.

The economical analysis of SHDHW system revealed that the concrete collectors provided about 49 and 63% of the annual load when the collecting area of the solar panel increased from 55 to 88 M² (25 to 40% of the building roof area). The corresponding solar contributions when conventional metallic collectors were used are 41 and 53%, respectively. This represents an improvement of the annual solar fraction of about 19% when concrete collectors are used instead of the metallic collectors.

It was found that solar heating systems with concrete solar collector models gave higher solar fractions and total life cycle savings than the conventional solar metallic collector.     

Experimental studies on a ground coupled heat pump with solar thermal collectors for space heating

This paper presents experimental studies on a solar-assisted ground coupled heat pump (SAGCHP) system for space heating. The system was installed at the Hebei Academy of Sciences in Shijiazhuang (lat. N38°03′, long. E114°26′), China. Solar collectors are in series connection with the borehole array through plate heat exchangers. Four operation modes of the system were investigated throughout the coldest period in winter (Dec 5th to Dec 27th). The heat pump performance, borehole temperature distributions and solar colleting characteristics of the SAGCHP system are analyzed and compared when the system worked in continuous or intermittent modes with or without solar-assisted heating. The SAGCHP system is proved to perform space heating with high energy efficiency and satisfactory solar fraction, which is a promising substitute for the conventional heating systems. It is also recommended to use the collected solar thermal energy as an alternative source for the heat pump instead of recharging boreholes for heat storage because of the enormous heat capacity of the earth.     

Comparison of the performance of two solar heating systems used to improve the microclimate of agricultural greenhouses in Morocco

This study presents an experimental investigation of two different types of solar systems used for heating agricultural greenhouses. The first system employs solar flat plate collectors, installed outside a polyethylene covered greenhouse. The second system, based on the selective absorption of solar energy by a heat transfer fluid (blue of methylene), employs polyethylene alveolar transparent plan collectors, installed inside a glass covered greenhouse. Experiments on both systems were simultaneously conducted, over several days. A comparative study of the performance of both systems was carried on the experimental results achieved in the plastic greenhouse heated by the first system, and on the simulated results obtained for the same greenhouse, but heated by the second system.     

Experimental study with operational solar‐sorption cooling

Building integrated solar systems have been considered as a reasonable system for building heating, cooling and hot water supply. Various types of solar collectors, such as plate type, evacuated tube type and solar air collector, have been used as the heat source, whereas adsorption chillers, absorption chillers and desiccant dehumidification systems have been considered to match the above solar heat sources. Now, such sorption chillers are more matured, but their coupling with suitable solar heat source is not well researched. Experimental study has been done in this paper to analyse four kinds of typical solar air‐conditioning system with different sorption chillers and solar collectors. Copyright © 2012 John Wiley & Sons, Ltd.     

Swedish solar heated residential area with seasonal storage in rock: Initial evaluation

A partly solar heated building area comprising 50 residential units has been built in Anneberg, Sweden. The system includes low-temperature space heating with seasonal ground storage of solar heat. Heating is supplied by 2400 m² solar collectors and individual electrical heaters for supplementary heating. The ground storage comprises about 60,000 m³ of crystalline rock with 100 boreholes drilled to 65 m depth and fitted with double U-pipes. The collectors will have favourable working conditions but the store is rather small, the estimated heat loss from the heat store is about 40% of stored solar heat and the average solar fraction is estimated to 70% after 3–5 years of operation. An initial evaluation after 2 years of operation shows that, although problems have occurred and several parts seem to work less efficient than expected, the overall system idea works as intended.     

A solar thermal cooling and heating system for a building: Experimental and model based performance analysis and design

A solar thermal cooling and heating system at Carnegie Mellon University was studied through its design, installation, modeling, and evaluation to deal with the question of how solar energy might most effectively be used in supplying energy for the operation of a building. This solar cooling and heating system incorporates 52 m² of linear parabolic trough solar collectors; a 16 kW double effect, water-lithium bromide (LiBr) absorption chiller, and a heat recovery heat exchanger with their circulation pumps and control valves. It generates chilled and heated water, dependent on the season, for space cooling and heating. This system is the smallest high temperature solar cooling system in the world. Till now, only this system of the kind has been successfully operated for more than one year. Performance of the system has been tested and the measured data were used to verify system performance models developed in the TRaNsient SYstem Simulation program (TRNSYS). On the basis of the installed solar system, base case performance models were programmed; and then they were modified and extended to investigate measures for improving system performance. The measures included changes in the area and orientation of the solar collectors, the inclusion of thermal storage in the system, changes in the pipe diameter and length, and various system operational control strategies. It was found that this solar thermal system could potentially supply 39% of cooling and 20% of heating energy for this building space in Pittsburgh, PA, if it included a properly sized storage tank and short, low diameter connecting pipes. Guidelines for the design and operation of an efficient and effective solar cooling and heating system for a given building space have been provided.     

Design and performance of the solar-powered floor heating system in a green building

In the green building of Shanghai Research Institute of Building Science, the evacuated tubular solar collectors with a total area of 150 m² were installed to provide heating for the covered area of 460 m². The floor heating coil pipes were made of high-quality pure copper with the dimension of Φ 12 × 0.7 mm. Under typical weather condition of Shanghai, the average heating capacity was 25.04 kW during the working hours from 9:00 to 17:00, which was sufficient to keep indoor thermal environment. The average electric COP of the floor heating system was 19.76 during the system operation. Compared with the widely used air-source heat pump heating systems with the electric COP of 3.5 in Shanghai, the solar-powered floor heating system shows great potential in energy conservation in winter. With respect to the whole heating period, the solar fraction was 56%. According to the performance analysis of the system with ambient parameters, it was observed that the system performance could be greatly enhanced with the increase of daily solar insolation. However, the system performance varied slightly with average ambient temperature. Compared with average ambient temperature, daily solar insolation had a more distinct influence on the performance of the solar-powered floor heating system.     

Energy consumption minimization in an innovative hybrid power production station by employing PV and evacuated tube collector solar thermal systems

Combination of a grid connected photovoltaic (PV) plant with a compressed air energy storage system (CAES) and a city gate station (CGS) has been proposed and investigated recently, leading to satisfactory performance results. The only deficiency of this system is the huge amount of fuel required to provide its heating demand. In this work, feasibility of employing evacuated tube solar thermal systems to supply the heating demand of the hybrid power plant is studied. After presenting detailed mathematical modeling, the solar heating units and other components of the power plant are properly sized. The results of simulations demonstrate that a total of 7000 evacuated tube collectors are required in the system, leading to elimination of the air heater from the CAES system completely and 17.2% fuel saving at the CGS. The total annual solar heat of 17.5 GWh is supplied for the system, 214 GWh power could directly be sold to the grid, 9.7 GWh power slumps is recovered and 53.5 GWh power is produced at nights. In the end, internal rate of return (IRR) method is used to compare economically the proposed system with similar systems proposed previously, outperforming all of the other candidates with an IRR of 11.1%.     

Flat-plate solar-collector performance evaluation with a solar simulator as a basis for collector selection and performance prediction

The use of a solar simulator for performance determination permits collector testing under standard conditions of wind, ambient temperature, flow rate and “Sun”. The performance results determined with the simulator have been found to be in good agreement with outdoor performance results.This paper reports the measured thermal efficiency and evaluation of 23 collectors which differ according to absorber material (copper, aluminum, steel), absorber coating (nonselective black paint, selective copper oxide, selective black nickel, selective black chrome), type of glazing material (glass, Tedlar, Lexan, anti-reflection glass), the use of honeycomb material and the use of vacuum to reduce thermal convection losses. The collectors are given performance rankings based on noon-hour solar conditions and all-day solar conditions. The determination with the simulator of an all-day collector performance is made possible by tests at different incident angles. The solar performance rankings are made based on whether the collector is to be used for pool heating, hot water, absorption air conditioning, heating, or for a solar Rankine machine.Another test which aids in selecting collectors is a collector heat capacity test. This test permits a ranking of collectors according to their heat capacity (and time constant), which is a measure of the rapidity of a collector's response to transient solar conditions. Results are presented for such tests.Final considerations for collector selection would of course be made on the basis of cost and the reliability of performance over the required life of a collector. Results of a cost-effectiveness study is given for conditions corresponding to those required for absorption or heating. These results indicate that the additional cost involved in the upgrading of collector performance (selective surfaces, anti-reflection glass, etc.) appears to be cost effective and therefore justified. Some data are also presented to illustrate a method for the determination of outdoor performance degradation by use of simulator tests carried out before and after a period of outdoor operation.