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 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%.     

Solar ponds for space heating

Solar ponds are shallow bodies of water in which an artificially maintained salt concentration gradient prevents convection. They combine heat collection with long-term storage and can provide sufficient heat for the entire year. We consider the absorption of radiation as it passes through the water, and we derive equations for the resulting temperature range of the pond during year round operation, taking into account the heat that can be stored in the ground underneath the pond. Assuming a heating demand of 25000 Btu/degree day (Fahrenheit), characteristic of a 2000 ft² house with fair insulation, and using records of the U.S. Weather Bureau, we carry out detailed calculations for several different locations and climates. We find that solar ponds can supply adequate heating, even in regions near the arctic circle. In midlatitudes the pond should be, roughly speaking, comparable in surface area and volume to the space it is to heat. Under some circumstances, the most economical system will employ a heat pump in conjunction with the solar pond. Cost estimates based on present technology and construction methods indicate that solar ponds may be competitive with conventional heating.     

DESIGN AND PERFORMANCE EVALUATION OF A SOLAR POND FOR INDUSTRIAL PROCESS HEATING

This paper describes the design of a solar pond for delivering 54 m³/day of hot water at 60°C to a catering facility in Singapore. The design of the pond was carried out in two steps. First, the depths of different layers of the pond were determined by considering the maximum temperature of the storage zone and the useful energy gain. For the given load and the local meteorological conditions, the optimum depths of various layers of the pond were found as follows:

Depth of surface-mixed layer:0.32 m

Depth of the insulation zone :1.00 m

Depth of storage zone:1.00 m

Total depth of the pond:2.32 m

The minimum payback period was used as the economic figure of merit to determine the optimum area of the pond. The optimum area of the pond is 6000 m². The payback period depends on the transparency of the pond, and for the conditions considered in the study, it varies between 3 and 4.5 years, The solar fraction varies from 65% for extinction coefficient, ( μ = 1.0m^?1 to 94% for μ = 0.55 m^?1, An experimental pond with an area of 14 m² and a depth of 1.5 m was built and tested over a period of time under the meteorological condition of Singapore. These results are used to validate the mathematical equations used in the design of the solar pond. A good agreement was found to exist between experimental and analytical results.     

Transparent Insulation System for Passive Solar Energy Utilization in Buildings

A new concept for the passive use of solar energy, transparent insulation, is described together with the first experimental results. Transparent insulation material has the property of being transparent or translucent to solar radiation while at the same time acting as heat insulation, Elements made of this material can be attached to the walls of buildings and thus permit the utilization of solar energy for heating. Relations are given for the dependence of heat flux and conversion efficiency of radiation into useful heat on the thermal resistance of the components. Calculations using meteorological data show that with materials parameters achievable with present technology not only south but also west/east and possibly even north orientations can lead to significant contributions to heating. In order to avoid overheating in summer, control of radiation must be provided.

Experiments with unoptimized materials on two buildings during 1982/83 showed promising results: for a south facade during a Sunny period in January, a mean flux of 9 W/m² into the house was observed. For the entire heating season the extrapolated value is 16 W/m². On a western orientation a net loss was observed, but the mean effective heat transfer coefficient of the element was lowered from 1.8 W/m²K without radiation to 0.25 W/m²K with radiation. The beneficial effect of masonry walls with regard to heat storage and damping of temperature fluctuation was also demonstrated. The elements proposed here therefore appear particularly attractive for retrofitting.     

Latent heat storage applied to horticulture

A new concept of solar heat storage applied to horticulture has been tested through a full heating season in a professional greenhouse. This experiment is run in a 500 m² multi-span, single-glazed greenhouse of traditional geometry devoted to rose production. The solar heat available inside the greenhouse is transferred and stored by recycling the air through an underground network of flat heat exchangers filled with a phase change material. In an attempt to reach quasi autonomy in a mild climate, the glass is doubled on the inside with an insulating polyethylene Air-Cap (R) film.The solar greenhouse compartment is compared with a traditional greenhouse compartment of identical geometry bearing the same plantation. The cover of this control compartment is single glazed. Thermal and cultural performances were analysed from December 1979 to April 1980. The solar compartment achieved 80 per cent savings in propane gaz, compared with the control compartment run at the same temperature. Compared with a control compartment doubled with Air-Cap polyethylene, the savings would reduce to 60 per cent. The electrical consumption of the fans of the solar system amount to less than 10 per cent of the basic heating loads.The solar prototype cost twice as much as the control compartment which compares favorably with other solar prototypes known at this time.     

A technoeconomic simulation model for A hybrid solar water heating system

A simple mathematical model has been developed to evaluate the technoeconomic performance of a hybrid solar water heating system for commercial and industrial applications. Numerical calculations, corresponding to Delhi climatic data and for the prevalent cost of a solar energy system in the Indian market, show that the optimum collector area (meeting nearly 45 percent of the daily hot water demand M litres) is 0–0075 Mm²; either a reduction of about 35 per cent in the present solar collector costs or a more than 20 per cent rise in the cost of presently subsidized diesel oil makes the solar option economic. With the present parameters the cost of useful solar energy is higher than that obtained from the conventional system.     

Field testing on nonsalt solar ponds

A new type of nonsalt solar pond was investigated by field testing. The roof of the solar pond was formed using a transparent double film. Three kinds of tests were carried out under the following conditions: (1) insulating pellets were packed between the layers of the transparent double film of the roof at sunset; (2) the water surface of the pond was insulated using only the two transparent films; (3) the water surface of the pond was covered by the double film with the top surface blackened on which solar energy can be collected, while pond water was circulated using a solar cell powered submerged water pump. The warm water stored in the solar pond by the above methods was utilized as a heat source for a gas engine powered heat pump used to heat a greenhouse. In this report, the results of the field tests on the above solar ponds and greenhouse heating system are discussed. Also the utility of a combination plant using a solar pond and underground borehole storage system is evaluated.Important conclusions on performance are as follows: (1) collection efficiencies of these solar ponds become 9–54% corresponding to the weather conditions and pond temperatures; (2) maximum temperature of the pond water under weather conditions at Osaka is about 80°C; (3) the solar pond can be effectively utilized for heating a greenhouse; (4) the combination plant using the solar pond and the underground storage layer can store heat of 1119 MJ m⁻² yr⁻¹.     

Dependence of ground heat loss upon solar pond size and perimeter insulation calculated and experimental results

Ground heat losses from solar ponds are modelled numerically for various perimeter insulation strategies and several solar pond sizes. The numerical simulations are steady state calculations of heat loss from a circular or square pond to a heat sink at the outer boundaries of an earth volume that surrounds the pond on the bottom and sides. Simulation results indicate that insulation on top of the ground around the pond perimeter is rather ineffective in reducing heat loss, and that uninsulated sloping side walls are slightly more effective than insulated vertical side walls, except for very small ponds. The numerical results are used to derive coefficients for a semi-empirical equation describing ground heat loss as a function of pond area, pond perimeter and insulation strategy. Experimental results for ground heat loss and energy balance in the 400 m² solar pond at the Ohio State University are reported. Analysis of this data, along with data on solar energy input, heat gain by the pond, heat loss through the gradient zone, and heat extraction from the pond yields a good energy balance. Numerical simulation of ground heat loss from this pond shows good agreement with the results obtained from pond measurements. Loss turns out to be large because of unexpectedly high values of earth thermal conductivity in the region.     

Constant flow solar pond collector/storage system

This paper presents a periodic analysis of the process of heat extraction by the brine layer circulating at constant flow rate through the bottom convective zone of a solar pond. Explicit expressions for the transient rate of heat extraction and the temperature at which heat can be extracted, as a function of time, depths of convective as well as non-convective zones and the flow rate, are derived. Extensive analytical results for the optimum performance of a pond during its year round operation are presented. In a pond with an upper convective zone depth of 0.2 m optimum heat extraction efficiencies of 24 per cent, 29 per cent and 32 per cent corresponding to heat extraction temperatures of 89, 55 and 42°C are predicted for water flow rates of 2 × 10^?4, 5 × 10^?4 and 10^?3 kg/s m², respectively. The load levelling in the extracted heat flux as well as in its temperature improves as the flow rate is lowered and the non-convective zone is over sized. An increase in the total depth of the solar pond improves the load levelling in extraction temperature, but influences the extracted heat flux differently; shifts its maximum to winter months and deteriorates the load levelling. The variability in flow rate required for the maintenance of constant temperature of the heat extraction zone is also investigated. It is found that the required variability is less for higher temperatures of the heat extraction zone and larger depths of the non-convective zone.     

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.     

Performance evaluation of a small‐scale sodium carbonate salt gradient solar pond

In this study, thermal performance of the salt gradient solar pond (SGSP), which of density gradient is artificially with sodium carbonate solution, was tested under Karabuk prevailing weather conditions in Turkey. A small‐scale prismatic glass tank was constructed with an area of 0.45 × 0.20 m² and a depth of 0.25 m as solar pond. A series of experiments with four different density levels were conducted in July–August 2004. The variations of the temperature and density profiles were observed for each of experiment for a week. It was found that the maximum temperature difference between the bottom and surface of the pond is 21°C and maximum temperature in the lower convective zone (LCZ) has been measured as 49°C at the first experiment. The efficiency of the pond was evaluated 13.33% weekly mean radiation intensity of 524 W m^?2 for the first experiment. Copyright © 2006 John Wiley & Sons, Ltd.     

Meeting the electricity demand for the heating of greenhouses with hydrogen: Solar photovoltaic-hydrogen-heat pump system application in Turkey

Providing the heating system with coal in greenhouses causes harmful results in terms of carbon emissions. In this study, analyzes were performed to meet the electrical energy required for the heating system with a heat pump from a solar photovoltaic-hydrogen system. For floor area 25000 m² where greenhouses the required energy is obtained directly from hydrogen without using a heat pump 3000 m² solar panel area required. The use of a heat pump reduces energy needs but it is also not feasible for large greenhouses. For convenience, a solar photovoltaic-hydrogen-heat pump system analysis was also made for 1000 m² floor area greenhouses and it is found that the 24 m² solar panel area is adequate in terms of meeting energy demand. Using a solar-hydrogen-heat pump system reduces carbon emissions by 86.5 tons per 1000 m² floor area greenhouse. Considering the hydrogen storage system becomes unfeasible. We normalized the greenhouse floor area to 1 m² and proposed reference values for hydrogen to be produced in 1 h, storage, and PV area. In addition, an analysis was made for the use of hydrogen energy for greenhouses that do not require a heating system and only work with a water pump.     

A feasibility study of solar heating in Iran

A single-glass, flat-plate solar collector for air heating is analyzed for an optimum tilt angle of 45° for Shiraz (29° 36′ N latitude, 52° 32′ E longitude, and elevation of 4500 ft). The absorbed and utilized solar energy, as well as the collector outlet air temperature, the glazing, and the blackened plate temperatures, are determined with respect to the incident solar energy, parametric with collector inlet air temperatures and flow rates and outside air temperature.A 10 ft² collector and an 8 ft³ rock storage are built to experimentally verify the analysis and obtain cost estimates. A 5000 ft² single-story building is considered for solar heating and economic evaluations. Based on an annual interest rate of 8 per cent amortization of the solar heating equipment over 15 yr, electrical energy costs of 3c/kWh, and fuel costs of $1·10 per 10⁶ B.t.u., the optimum collector area which results in minimum annual operating costs (of the solar heating system and the auxiliary heating unit) is determined. A net saving results because solar heating is employed. The feasibility study is extended to eleven other Iranian cities. It is found profitable to employ solar heating in cities with low annual rainfall and relatively cold winters. An effective evaporative cooling is obtained by spraying water over the rock storage during the summer.     

Development of the gel pond technology

A review of the development of the gel pond technology is presented. First, the emergence and growth of solar pond technology since the 1950's is described. The inherent problems encountered with the conventional salt gradient ponds are discussed, leading to the concept of the solar gel pond in which the salt gradient layer in the former is replaced by a transparent polymer gel. The major work in the first phase dealt with the experimental development of a polymer gel which met certain selection criteria. The criteria considered included transmissivity, stability of physical and chemical properties, high viscosity and other physical and optical properties. The gradual development of the polymer gel through an alternating process of testing and elimination and evaluation of relevant properties of the gel has been described. Modeling and optimization studies of the solar gel pond have been presented. Bansal and Kaushik's model for a salt gradient pond has been modified for a solar gel pond, and the results of the simulation are presented in a graphical form to serve as a quick reference for estimation of pond surface area, depth and flow rate for heat extraction depending on the extreme temperature required in the storage zone and the required heat load. Then, a cost-benefit economic analysis compares the economics of a solar gel pond with a conventional salt gradient pond. The construction of an experimental gel pond (18 m²) at The University of New Mexico is described, and the results of the study are summarized. Information on commercial scale ponds at Chamberino, New Mexico (110 m²), and in Albuquerque, New Mexico (400 m²), is provided. The review of the technology demonstrates the immense potential of the gel pond as a source of alternate energy for the years ahead.     

Application demonstration and performance of a cellulose triacetate polymer film based transparent insulation wall heating system

For the application demonstration of cellulose triacetate (CTA) polymer film based transparent insulation (TI) structures a technically and ecologically optimized TI facade system was developed and used to equip a south-oriented wall of a solar house meeting passive house standard in Graz, Austria. The demonstration building was equipped with an appropriate data recording system for solar irradiation, temperature, heat flux and humidity. The practical experiences within the heating periods 2002/03 and 2003/04 are reported in this paper. For the optimized TI facade system a solar energy efficiency of about 43% and a U-value of 0.76 W/(m² K) were obtained. Although CTA absorbs a high amount of water no adverse condensation phenomena were observable visually. The reasoning for these findings is explained and related to construction details.     

Simple multiple wick solar still: Analysis and performance

This paper presents the design, analysis and performance of a multiple wick solar still, in which blackened wet jute cloth forms the liquid surface which can be oriented to intercept maximum solar radiation and attain high temperatures on account of low thermal capacity. The wet surface consists of a series of jute cloth pieces of increasing length separated by thin black polythene sheets, resting on foam insulation supported by a net of nylon ribbon; these pieces are arranged along an incline and their upper edges are dipped in a saline water tank. Suction by the cailliary action of the cloth fibre provides a thin sheet of liquid on the cloth; the arrangement ensures that all the surface, irradiated by the sun is wet at all times. The results of an analysis based on Dunkle's relation[16] are in excellent agreement with the observed performance of the still. On a typical cold sunny day in Delhi (viz. 6 February 1980) the distillate output was 2.5 l/m² day, corresponding to an overall efficiency of 34 per cent (as compared to a maximum of 30 per cent for basin type still). The still costs less than half of the cost of a basin type still of same area and provides a higher yield of distillate.     

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.     

Performance at Colombo, Ceylon, of a pressurised solar water heater of the combined collector and storage type

A solar water heater combining collection and storage has been tested in Ceylon. The heater consists of a square coil of 3 in. diameter pipe, 44·3 ft in length, in a wooden box with heat insulation at the bottom and two glass covers. The glass surface is 20 ft² in area. If water is drawn off whenever the water reaches 120°F, it is possible to obtain 30–50 gal of water a day, the first draw-off being made about noon. The efficiency of collection based on the exposed glass area on the top of the box (which was 1·55 times larger than the horizontally-projected area of the pipe surface) is around 46 per cent.An equation for the performance of the heat collector over the day was devised, and this was applied by means of a computer program to solar radiation data for the whole year. The results showed that no heat could be collected for less than 10 per cent of the year. The total energy collection for the year was about 1250 kWh.Another computer program compared the performance of the heaters made up of 2, , 3, and 4 in. diam pipes respectively. This study indicated that a heater of to in. diam pipe would provide the best performance for most purposes.     

Solar water heating in Lebanon: Current status and future prospects

The use of solar thermal collectors is an economic alternative for water heating in Lebanon. More than 100,000 m² of collector area has been installed while the market can accommodate more than 1.5 million m². The domestic sector, which is a main energy-consuming sector, stands to benefit the most from the implementation of such systems. Despite the lack of encouraging legislation, the solar thermal market has been continuously growing over the past decade. Both local manufacturers and importers have been active in the field. In addition, advanced forced circulation and collective systems are being used in large establishments, individual house and apartment buildings. Internationally funded demonstration projects using collective systems have been implemented in recent years with promising results. Simplified initial estimates indicate a payback period of 4–5 years while advanced mathematical models (RETScreen) indicate that the most advanced evacuated tube technology has a payback period of less than 9 years at current market prices. With decreasing cost per square meter of installed collectors, payback periods are expected to rapidly decrease. Regulatory support and tax breaks, if implemented, will have a positive effect on the market. The current increases in diesel prices are increasing demand on solar thermal water heaters.