A numerical model for seasonal storage of solar heat in the ground by vertical pipes

We present a three-dimensional numerical model for seasonal heat storage in the ground using vertical heat exchanger pipes. The model also accounts for convective heat flows in the ground. The storage is employed in a district solar heating system with a heat pump. The effects of storage volume, storage medium, collector area, and collector type on system performances are studied for the Helsinki (60°N) climate. Economic optimization of the storage and collector installation is also briefly discussed. For a 500-house community, a collector area of 35 m² per house and a rock storage volume of 550 m³ per house would provide a solar fraction of 70%.     

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.     

Methodology for estimation of potential for solar water heating in a target area

Proper estimation of potential of any renewable energy technology is essential for planning and promotion of the technology. The methods reported in literature for estimation of potential of solar water heating in a target area are aggregate in nature. A methodology for potential estimation (technical, economic and market potential) of solar water heating in a target area is proposed in this paper. This methodology links the micro-level factors and macro-level market effects affecting the diffusion or adoption of solar water heating systems. Different sectors with end uses of low temperature hot water are considered for potential estimation. Potential is estimated at each end use point by simulation using TRNSYS taking micro-level factors. The methodology is illustrated for a synthetic area in India with an area of 2 sq. km and population of 10,000. The end use sectors considered are residential, hospitals, nursing homes and hotels. The estimated technical potential and market potential are 1700 m² and 350 m² of collector area, respectively. The annual energy savings for the technical potential in the area is estimated as 110 kW h/capita and 0.55 million-kW h/sq. km. area, with an annual average peak saving of 1 MW. The annual savings is 650-kW h per m² of collector area and accounts for approximately 3% of the total electricity consumption of the target area. Some of the salient features of the model are the factors considered for potential estimation; estimation of electrical usage pattern for typical day, amount of electricity savings and savings during the peak load. The framework is general and enables accurate estimation of potential of solar water heating for a city, block. Energy planners and policy makers can use this framework for tracking and promotion of diffusion of solar water heating systems.     

OPTIMIZATION OF COLLECTOR AREA IN SOLAR WATER HEATING SYSTEMS

Domestic household thermosyphons are economically feasible and are used by over than 70% of houses in Palestine. Although domestic solar water heating for commercial applications has a good potential, only a few systems have been installed in Palestine. A systematic sizing approach for the solar system is presented in this paper and applied to a certain case study. The solar system sizing is based on the life-cycle cost LCC analysis. For the chosen case study of domestic water heating for a hotel, with hot water consumption of 2600 liters per day, the optimum collector area was found to be 37 m², the solar fraction of heating 0.78, the LCC of system is SI 3778, with annual savings of 1338$/year and a pay back period of 3 years. With this optimized system, the cost of water heating is 1.8 $/m³comparing with 2.6 $/m³ for the conventional system.     

The energy saving obtainable with solar heating and heat pumps in a northern climate

The energy saving obtainable with active solar heating and heat pumps has been studied for several years in the Northern climate of Finland. The studies deal mainly with small houses. A computer program is developed which calculates hour by hour the annual energy balance of different heating systems. The performance, of the heating systems are also measured in inhabited houses. The calculations show that the useful solar energy obtainable from the collector is 50–400 kWh/m² annually depending on the system and the collector size. A heat pump in the system is very advantageous, because it keeps the heat losses low and the collector efficiency high. It approximately doubles the energy obtainable. The measurement results have not been as good as expected. The solar energy obtained from the collector has been 120–160 kWh/m² annually. The main reasons for the low solar energy are design and equipment faults and the shading effects. The best energy saving device is the earth heat pump. It is also therefore very advantageous that the peak power demand decreases markedly. When the area of the earth pipes is large enough, energy may be extracted from earth through the whole year. The annual coefficient of performance is 2–3. Also a heat pump which extracts heat from exhaust air in dwelling houses has been very promising.     

1 MWth INDUSTRIAL SOLAR HOT WATER SYSTEM AND ITS PERFORMANCE

An industrial model solar water heating system is designed and installed, to heat and supply 110 000 liters of hot water at 85°C per day for an egg powder making plant. It consists of a solar collector field (2560 m²), four thermally insulated hot water storage tanks (57.5 m³ capacity each) and the heat distribution system with electrically operated pumps and pneumatic valves for circulation of water. It is equipped with a PC based data acquisition system to monitor the process parameters, a fault detection system to monitor the status of various subsystems and controls for automatic operation of the system. Performance studies conducted on the various subsystems and on the system as a whole revealed that it is delivering the designed thermal output, and the net savings in furnace oil consumption is 78% on an annual basis.     

Solar space heating and cooling for Spanish housing: Potential energy savings and emissions reduction

An experimental solar energy facility was designed to meet as much of the heating demand in a typical Spanish dwelling as possible. With a view to using the facility during the summer and preventing overheating-induced deterioration of the solar collectors in that season of the year, an absorption chiller was fitted to the system to produce solar-powered air conditioning. The facility operated in solar space heating mode in the winter of 2008–2009 and in cooling mode during the summer of 2008. The design was based on a new type of flat plate vacuum solar collectors that delivered higher efficiency than conventional panels. This type of collectors can reach temperatures of up to 110 °C in the summer and up to 70 °C on the coldest winter days. The solar facility comprised a 48-m² (with a net area of 42 m²) solar collector field, a 25-kW plate heat exchanger, a 1500-l storage tank, a 4.5-kW (Rotartica) air-cooled absorption chiller and several fan coils. The facility was tested by using it to heat and cool an 80-m² laboratory located in Madrid. As the average area of Spanish homes is 80 m², the findings were generally applicable to national housing. The solar facility was observed to be able to meet 65.3% of the space heating demand. For air conditioning, the system covered 46% of the demand, but with high indoor temperatures. In other words, the collector field was found to be able to air condition only half of the home (40 m²). Lastly, the savings in CO₂ emissions afforded by the use of this facility compared to conventional air conditioning were calculated, along with its amortisation period. These results have been extrapolated calculating the potential energy savings and emissions reduction for all the Spanish households.     

Parabolic trough collectors for industrial process heat in Cyprus

The thermal utilization of solar energy is usually confined to domestic hot water systems and somewhat to space heating at temperatures up to 60 °C. Industrial process heat has a considerable potential for solar energy utilization. Cyprus has a small isolated energy system, almost totally dependent on imported fuels to meet its energy demand. The abundance of solar radiation together with a good technological base, created favorable conditions for the exploitation of solar energy in the island. The number of units in operation today corresponds to one heater for every 3.7 people in the island, which is a world record. Despite this impressive record no solar industrial process heat system is in operation today. The main problem for this is the big expenditure required for such a system and the uncertainty of the benefits. The objective of this work was to investigate the viability of using parabolic trough collectors for industrial heat generation in Cyprus. The system is analyzed both thermally and economically with TRNSYS and the TMY for Nicosia, Cyprus, in order to show the magnitude of the expected benefits. The load is hot water delivered at 85 °C at a flow rate of 2000 kg/h for the first three quarters of each hour from 8:00–16:00 h, 5 days a week. The system consists of an array of parabolic trough collectors, hot water storage tank, piping and controls. The optimum collector area for the present application is 300 m², the optimum collector flow rate is 54 kg/m² h and the optimum storage tank size is 25 m³. The system covers 50% of the annual load of the system and gives life cycle savings of about C£6200 (€10800). This amount represent the money saved from the use of the system against paying for fuel. The savings however refer to a non-subsidized fuel price, which will be in effect from 2003. The optimum system can deliver a total of 896 GJ per year and avoids 208 tons of CO₂ emissions to the atmosphere. The effect of various design changes on the system performance was investigated. The E–W tracking system (collector axis aligned in N–S direction) was found to be superior to the N–S one. The required load temperature affects the performance of the system as for higher temperatures the auxiliary energy required is bigger. Also a number of variations in the load use pattern have been investigated and presented in this paper. It was found that the bigger the load (double shift, full hour use pattern) the bigger the collector area required, the greater the first year fuel savings and the greater the life cycle savings of the installation. This means that it is more viable to apply solar industrial process heat to higher energy consumption industries.     

Simulation of a solar heating and cooling system

This paper presents thermal and economic analyses of a solar heated and air conditioned house in the Albuquerque climate. The system includes the following components: water heating collector, a water storage unit, a service hot water facility, a lithium bromide-water air conditioner (with cooling tower), an auxiliary energy source, and associated controls. The analysis of the thermal performance indicates the dependence of output on collector area (considered as the primary design variable) and shows, for example, the manner in which annual system efficiency decreases as collector area increases. Based on the computed thermal performance, cost estimates are made which show variations in annual cost as functions of collector area and costs of collector and fuel.     

System performance and economic analysis of solar-assisted cooling/heating system

The long-term system simulation and economic analysis of solar-assisted cooling/heating system (SACH-2) was carried out in order to find an economical design. The solar heat driven ejector cooling system (ECS) is used to provide part of the cooling load to reduce the energy consumption of the air conditioner installed as the base-load cooler. A standard SACH-2 system for cooling load 3.5 kW (1 RT) and daily cooling time 10 h is used for case study. The cooling performance is assumed only in summer seasons from May to October. In winter season from November to April, only heat is supplied. Two installation locations (Taipei and Tainan) were examined.It was found from the cooling performance simulation that in order to save 50% energy of the air conditioner, the required solar collector area is 40 m² in Taipei and 31 m² in Tainan, for COP_j = 0.2. If the solar collector area is designed as 20 m², the solar ejector cooling system will supply about 17–26% cooling load in Taipei in summer season and about 21–27% cooling load in Tainan. Simulation for long-term performance including cooling in summer (May–October) and hot water supply in winter (November–April) was carried out to determine the monthly-average energy savings. The corresponding daily hot water supply (with 40 °C temperature rise of water) for 20 m² solar collector area is 616–858 L/day in Tainan and 304–533 L/day in Taipei.The economic analysis shows that the payback time of SACH-2 decreases with increasing cooling capacity. The payback time is 4.8 years in Tainan and 6.2 years in Taipei when the cooling capacity >10 RT. If the ECS is treated as an additional device used as a protective equipment to avoid overheating of solar collectors and to convert the excess solar heat in summer into cooling to reduce the energy consumption of air conditioner, the payback time is less than 3 years for cooling capacity larger than 3 RT.     

Feasibility study of a solar-assisted heating/cooling system for an aquatic centre in hot and humid climates

This paper reports on a feasibility study of a solar-powered heating/cooling system for a swimming pool/space combination in a tropical environment. The system utilizes an absorption chiller and a cooling tower to meet the facilities and locker room load. The heating is accomplished by employing hot water generated by heat exchange with the solar collector working fluid. Two thermal storage tanks were employed for the collector and domestic use. The absorption chiller utilizes hot water to regenerate the LiBr solution. The proposed system enables the swimming season to be extended from sixteen weeks to fifty-two weeks. Simulation results indicate that a combination of a double glazed collector area of 600–4800 m² and a storage tank volume of 11·36 m³ results in a 25–37% reduction in the consumption of natural gas. Economic analysis is performed based on the life-cycle-cost method and takes into account the effects of discount rate, fuel price and fuel inflation rate. Different scenarios for which the solar-assisted system is economical are presented and analysed. © 1997 John Wiley & Sons, Ltd.     

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.     

Potential benefits of cool roofs on commercial buildings: conserving energy, saving money, and reducing emission of greenhouse gases and air pollutants

Cool roofs—roofs that stay cool in the sun by minimizing solar absorption and maximizing thermal emission—lessen the flow of heat from the roof into the building, reducing the need for space cooling energy in conditioned buildings. Cool roofs may also increase the need for heating energy in cold climates. For a commercial building, the decrease in annual cooling load is typically much greater than the increase in annual heating load. This study combines building energy simulations, local energy prices, local electricity emission factors, and local estimates of building density to characterize local, state average, and national average cooling energy savings, heating energy penalties, energy cost savings, and emission reductions per unit conditioned roof area. The annual heating and cooling energy uses of four commercial building prototypes—new office (1980+), old office (pre-1980), new retail (1980+), and old retail (pre-1980)—were simulated in 236 US cities. Substituting a weathered cool white roof (solar reflectance 0.55) for a weathered conventional gray roof (solar reflectance 0.20) yielded annually a cooling energy saving per unit conditioned roof area ranging from 3.30 kWh/m² in Alaska to 7.69 kWh/m² in Arizona (5.02 kWh/m² nationwide); a heating energy penalty ranging from 0.003 therm/m² in Hawaii to 0.14 therm/m² in Wyoming (0.065 therm/m² nationwide); and an energy cost saving ranging from 0.126/m < sup > 2 < /sup > in West Virginia to0.126/m² in West Virginia to 1.14/m² in Arizona ($0.356/m² nationwide). It also offered annually a CO₂ reduction ranging from 1.07 kg/m² in Alaska to 4.97 kg/m² in Hawaii (3.02 kg/m² nationwide); an NO_x reduction ranging from 1.70 g/m² in New York to 11.7 g/m² in Hawaii (4.81 g/m² nationwide); an SO₂ reduction ranging from 1.79 g/m² in California to 26.1 g/m² in Alabama (12.4 g/m² nationwide); and an Hg reduction ranging from 1.08 μg/m² in Alaska to 105 μg/m² in Alabama (61.2 μg/m² nationwide). Retrofitting 80% of the 2.58 billion square meters of commercial building conditioned roof area in the USA would yield an annual cooling energy saving of 10.4 TWh; an annual heating energy penalty of 133 million therms; and an annual energy cost saving of $0.356/m² nationwide). It also offered annually a CO₂ reduction ranging from 1.07 kg/m² in Alaska to 4.97 kg/m² in Hawaii (3.02 kg/m² nationwide); an NO_x reduction ranging from 1.70 g/m² in New York to 11.7 g/m² in Hawaii (4.81 g/m² nationwide); an SO₂ reduction ranging from 1.79 g/m² in California to 26.1 g/m² in Alabama (12.4 g/m² nationwide); and an Hg reduction ranging from 1.08 μg/m² in Alaska to 105 μg/m² in Alabama (61.2 μg/m² nationwide). Retrofitting 80% of the 2.58 billion square meters of commercial building conditioned roof area in the USA would yield an annual cooling energy saving of 10.4 TWh; an annual heating energy penalty of 133 million therms; and an annual energy cost saving of 735 million. It would also offer an annual CO₂ reduction of 6.23 Mt, offsetting the annual CO₂ emissions of 1.20 million typical cars or 25.4 typical peak power plants; an annual NO_x reduction of 9.93 kt, offsetting the annual NO_x emissions of 0.57 million cars or 65.7 peak power plants; an annual SO₂ reduction of 25.6 kt, offsetting the annual SO₂ emissions of 815 peak power plants; and an annual Hg reduction of 126 kg.     

Performance of unglazed solar ventilation air pre-heaters for broiler barns

Solar radiation is an interesting heat source for applications requiring a limited amount of energy, such as pre-heating cold fresh air used in venting livestock barns. The objective of this study was to evaluate the energy recovery efficiency of a solar air pre-heater consisting of an unglazed perforated black corrugated siding where the incoming fresh ventilation air picks up heat from its face and back. Installed on the southeast wall of two broiler barns located 40 km east of Montreal, Canada, the performance of solar air pre-heaters was monitored over 2 years. Sensors inside the barns monitored the temperature of the ambient air, that pre-heated by the solar collector and that exhausted by one of the three operating fans. An on-site weather station measured ambient air temperature, wind direction and velocity and radiation energy absorbed on a vertical plane parallel to the unglazed solar air pre-heaters. The measured vertical solar radiation value was used to evaluate the heat recovery efficiency of the unglazed solar air pre-heaters. Using data from the Varennes Environment Canada weather station located 30 km northwest, the solar sensors were found to measure the absorbed solar radiation with a maximum error of 7%, including differences in exterior air moisture. Unglazed, the efficiency of the solar air pre-heaters reached 65% for wind velocities under 2 m/s, but dropped below 25% for wind velocities exceeding 7 m/s. Nevertheless, the unglazed solar air pre-heaters were able to reduce the heating load especially in March of both years. Over a period starting in November and ending in March, the solar air heaters recovered an energy value equivalent to an annual return on investment of 4.7%.     

A thermo-economical optimization of a domestic solar heating plant with seasonal storage

In this study, the thermo-economic optimization analysis to determinate economically optimal dimensions of collector area and storage volume in domestic solar heating systems with seasonal storage is presented. For this purpose, a formulation based on the simplified P₁ and P₂ method is developed and solved by using MATLAB optimization Toolbox for five climatically different locations of Turkey. The results showed that the required optimum collector area in Adana (37 °N) for reaching maximum savings is 36 m²/house and 65 m²/house in Erzurum (39 °N) for same storage volume (1000 m³). The effects of collector efficiency on solar fraction and savings are investigated. The simulation results showed that the solar fraction and savings of the selective flat plate collector systems are higher than the other black paint flat plate collector systems.     

Solar seasonal thermal energy storage for space heating in residential buildings: Optimization and comparison with an air-source heat pump

ABSTRACT

This study evaluates the techno-economics of replacing an air-source heat pump (ASHP) system with a solar seasonal thermal energy storage (STES) system for space heating in Hangzhou, China. Three heating systems, solar STES, ASHP, and ASHP with short-term storage of solar energy, are developed using TRNSYS for a house with 240 m² of floor area. The ratio of tank volume to collector area (RVA) of the STES is optimized for the lowest equivalent annual cost over a lifespan of 20 y. The determined optimal RVA is 0.33 m³/m², although it depends on the system and electricity prices. The optimized STES reduces the electricity demand to 1,269 kWh (74% reduction). Despite the superior energy performance, the economic benefit is only possible with large STES systems, which enjoy low tank prices due to scale effects. The results suggest that policy support is needed for STES, where district scaling is not an option.     

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%.     

Optimal design of a forced circulation solar water heating system for a residential unit in cold climate using TRNSYS

An indirect forced circulation solar water heating systems using a flat-plate collector is modeled for domestic hot water requirements of a single-family residential unit in Montreal, Canada. All necessary design parameters are studied and the optimum values are determined using TRNSYS simulation program. The solar fraction of the entire system is used as the optimization parameter. Design parameters of both the system and the collector were optimized that include collector area, fluid type, collector mass flow rate, storage tank volume and height, heat exchanger effectiveness, size and length of connecting pipes, absorber plate material and thickness, number and size of the riser tubes, tube spacing, and the collector’s aspect ratio. The results show that by utilizing solar energy, the designed system could provide 83-97% and 30-62% of the hot water demands in summer and winter, respectively. It is also determined that even a locally made non-selective-coated collector can supply about 54% of the annual water heating energy requirement by solar energy.     

Solar space heating and cooling with bi-heat source heat pump and hot water supply system

This paper describes a specific heat pump system that can solve the problem of low heating capacity at a low ambient temperature—one of the largest problems in the air-source heat pump system.

In order to decrease the collector area required, the heat pump system is operated by the air-source during the daytime, but at night or at a very low ambient temperature it can be operated with hot water which has been produced by the collector in the daytime. The effect of the solar energy on the air-source heat pump system has many advantages in the moderate winter climate of Japan. The hot water supply system includes an auxiliary electric heater.

The experiment has been carried out with a prefabricated test house, which has been constructed in Nara with double glazed windows and high thermal insulation. The results of this experiment are that solar energy enhances the total electric energy savings, increases the heating capacity at low ambient temperature, and eliminates the need for reverse cycle defrosting operation, etc.     

Influence of store dimensions and auxiliary volume configuration on the performance of medium-sized solar combisystems

To increase the fractional energy savings achieved with solar thermal combisystems the store volume may be increased. Installation of large stores in single-family houses is, however, often limited by space constraints. In this article the influence of the store dimensions, as well as internal and external auxiliary volume configurations, are investigated for large solar water stores by annual dynamic TRNSYS simulations. The results show that store sizes up to 4 m³ may be used in solar heating systems with 30 m² collector area. It is further shown that well-insulated stores are rather insensitive to the geometry. Stores deviating from the conventional dimensions still yield high fractional energy savings. Furthermore, the simulations show that the performance of an internal auxiliary volume configuration in most cases exceeds that of a solution with an external auxiliary unit. The practical limitations of very thin auxiliary volumes must, however, be further investigated.