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.     

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.     

Central solar heating plants with seasonal storage in Germany

In the house building sector, central solar heating plants presently offer the most cost-favourable application of all possibilities of solar-thermal systems. By the integration of seasonal heat storage, more than 50% of the annual heating demand for space heating and domestic hot water can be supplied by solar energy. Since 1995, eight central solar heating plants with seasonal heat storage have been built in Germany within the governmental R&D-programme ‘Solarthermie-2000’. This report describes the technology of central solar heating plants and gives advice about planning and costs. The pilot and demonstration plants for seasonal heat storage already built in Germany are described in detail to give an idea about possible system design and applications of central solar heating plants.     

A series connected solar air heating system using the FMTC collector and the rolling cylinder heat store

An annual performance simulation has been carried out using the FMTC collector, the rolling cylinder heat store with Glauber's Salt, and a dwelling, all arranged in a series connected solar air heating system. The results suggest that a modest size system can readily carry the entire heating load of an energy efficient home in any geographical region of the U.S. The energy required to drive the system, i.e. rolling cylinder drive energy plus air blower drive energy, is always an important fraction of the total system output. In Boston 1958 weather the seasonal COP varies from 2.5 to 5.8 depending on the particular rolling cylinder design configuration selected. 90 per cent of the system drive energy is base load for the electric utility. There are no “coldest day peaks” resulting from electric backup heating. A system using the highest drive energy configuration may be sized for 99.5 per cent solar system heating using the same design chart at any northern region of the U.S.A. One system sized for Boston was evaluated at seven other geographical locations. The resulting house size was always in the acceptable range. This raises the possibility that one or a few standardized system sizes could serve an area as large as the U.S. Individualized solar system design calculations would not be necessary. Cloudy weather performance was noticeably improved in Boston (1958 weather). On a seasonal basis the FMTC collector delivered three times more heat than a typical flat plate (TFP) collector and 60 per cent of that heat was collected at insolation levels which were below the functional cut-off level of the TFP collector.     

Simulation and cost optimization of solar heating of buildings in adverse solar regions

In the present study a model has been developed which simulates the effects of hourly weather conditions on the performance and cost of a combined solar/conventional heating system for buildings in cold, cloudy climates. The model exhibits the effects of several system and cost parameters on combined system cost so that optimal designs can be determined.Performance and cost results are presented for 1971 Ottawa, Ontario, weather data. The economic analysis, which treats both collector and conventional system fuel costs parametrically, shows that solar heating of a typical house in cold, cloudy climates is economically competitive with fuel oil heating only if the price of oil rises to approximately 80¢/gal.     

Design, analysis and optimization of solar industrial process heat plants without storage

Solar industrial process heat applications which have a constant daytime load every day of the year permit optimal collector utilization, thus minimizing the cost of solar energy. In particular, one can avoid the cost and performance penalties associated with storage of solar heat, if the backup can operate at variable heating rate. Hence, these applications are particularly relevant for the near future when solar equipment is still relatively expensive and one is looking for solar applications with the lowest possible cost. This paper develops a method for designing and optimizing installations of this type, including a quick procedure for selecting the most cost effective collector and calculation of pumping energy. The economic optimum is shown to imply a slightly oversized collector field with dumping of excess energy during peak insolation. Using recently developed correlations for the annual energy collectible by the principal collector types, we state all results as explicit algebraic equations which are readily evaluated with a hand calculator. The accuracy is on the order of 3 percent if there is no uncertainty in the input variables. The method is illustrated by an example.     

High temperature solar heated seasonal storage system for low temperature heating of buildings

A preliminary study of a solar-heated low-temperature space-heating system with seasonal storage in the ground has been performed. The system performance has been evaluated using the simulation models TRNSYS and MINSUN together with the ground storage module DST. The study implies an economically feasible design for a total annual heat demand of about 2500 MWh. The main objective was to perform a study on Anneberg, a planned residential area of 90 single-family houses with 1080 MWh total heat demand. The suggested heating system with a solar fraction of 60% includes 3000 m² of solar collectors but electrical heaters to produce peak heating. The floor heating system was designed for 30°C supply temperature. The temperature of the seasonal storage unit, a borehole array in crystalline rock of 60,000 m³, varies between 30 and 45°C over the year. The total annual heating costs, which include all costs (including capital, energy, maintenance etc.) associated with the heating system, were investigated for three different systems: solar heating (1000 SEK MWh⁻¹), small-scale district heating (1100 SEK MWh⁻¹) and individual ground-coupled heat pumps (920 SEK MWh⁻¹). The heat loss from the Anneberg storage system was 42% of the collected solar energy. This heat loss would be reduced in a larger storage system, so a case where the size of the proposed solar heating system was enlarged by a factor of three was also investigated. The total annual cost of the solar heating system was reduced by about 20% to about 800 SEK MWh⁻¹, which is lower than the best conventional alternative.     

Determination of design space and optimization of solar water heating systems

In this paper, a methodology is proposed to determine the design space for synthesis, analysis, and optimization of solar water heating systems. The proposed methodology incorporates different design constraints to identify all possible designs or a design space on a collector area vs. storage volume diagram. The design space is represented by tracing constant solar fraction lines on a collector area vs. storage volume diagram. It has been observed that there exists a minimum as well as a maximum storage volume for a given solar fraction and collector area. Similarly existence of a minimum and a maximum collector area is also observed for a fixed solar fraction and storage volume. For multi-objective optimization, a Pareto optimal region is also identified. Based on the identified design space, the solar water heating system is optimized by minimizing annual life cycle cost. Due to uncertainty in solar insolation, system parameters and cost data, global optimization may not be utilized to represent a meaningful design. To overcome this, a region of possible design configurations is also identified in this paper.     

Performance analysis and cost optimization of a solar-assisted heat pump system

A solar-assisted heat pump system with a conventional backup unit was simulated for a 93 m² (1000 ft²) house in Rhode Island using quasi-dynamic computer models. The performance of the system as a function of collector area and thermal storage volume was evaluated to determine the fraction of the space heating and domestic hot water load that was supplied by the solar-assisted system. This information was used to compute the payback time, based on cumulative costs, for each variation of the system's parameters when compared to a conventional system. The optimal combination of system components which had a payback time less than the mortgage life was determined. For the given initial costs of solar panels and storage reservoir, this optimal combination was found to be insensitive to the variations in mortgage and fuel cost growth rates presented in this report.     

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%.     

Optimum storage of heat with a solar house

This paper reports calculations of the economic feasibility of solar house heating (and cooling) for different localities in the United States, and presents results in terms of maximum cost per square foot of solar collector which can be afforded if the solar system is to be competitive. Sunny mountainous regions, with cold winters (large fuel bills to be saved) are the most favorable areas.

Particular attention is given to the efficiency and capacity of energy storage and to the possibilities of saving summer heat for winter use. It is found that long-term storage is less economical than short-term storage. Storage costs of about a dollar per therm (10⁵ Btu), or solar collectors built and installed for $1.50 per sq ft, would make solar houses commercially competitive.     

Seasonal storage of energy in solar heating

This paper focuses on several aspects of seasonal storage for space heating using water as the storage medium. The interrelationships between collector area, storage volume, and system performance are investigated using the transient simulation program TRNSYS. The situations for which seasonal storage is most promising are presented. Particular emphasis is placed upon design of seasonal storage systems. A design method is presented which is applicable for storage capacities ranging from a few days to seasonal storage. This design method, coupled with cost information, should be useful in assessing the economic viability of seasonal storage systems. Also investigated are the importance of the load heat exchanger size, tank insulation, collector slope, and year-to-year weather variations in system design.     

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.     

A design procedure for solar air heating systems

A natural extension of the design procedure for liquid-based solar space and water heating systems is a similar analysis for solar heating systems using air as the heat transfer fluid. In this paper, a solar air heating system incorporating a flat-plate air heater and packed bed thermal storage is described and a simulation model for the system is developed. The results of many simulations of the air heating system are used to establish the relationship between system performance and the system design and meteorological variables. The results are presented in analytic and graphical form, referred to as an f-chart for solar air heating systems. The results of simulations in several widely different climates suggest that the information presented in the f-chart is location independent. Methods of estimating the performance of air heating systems having a collector air capacitance rate and a storage capacity other than those used to generate the f-chart are included. A comparison of the performance of air and liquid based systems is afforded by a comparison of their respective f-charts. The air system is shown to perform better at high load fractions supplied by solar energy than a liquid-based system with the same collector thermal performance parameters.     

Energy analysis and modeling of a solar assisted house heating system with a heat pump and an underground energy storage tank

An analytical model is presented and analyzed to predict the long term performance of a solar assisted house heating system with a heat pump and an underground spherical thermal energy storage tank. The system under investigation consists of a house, a heat pump, solar collectors and a storage tank. The present analytical model is based on a proper coupling of the individual energy models for the house, the heat pump, useful solar energy gain, and the transient heat transfer problem for the thermal energy storage tank. The transient heat transfer problem outside the energy storage tank is solved using a similarity transformation and Duhamel’s superposition principle. A computer code based on the present model is used to compute the performance parameters for the system under investigation. Results from the present study indicate that an operational time span of 5–7 years will be necessary before the system under investigation can attain an annually periodic operating condition. Results also indicate a decrease in the annually minimum value of the storage tank temperature with a decrease in the energy storage tank size and/or solar collector area.     

Prediction of the performance of solar heating systems over a range of storage capacities

A new method for prediction of the performance of solar heating systems using well-mixed storage is presented which predicts monthly and annual system performance (relative to a computer simulation) over a wide range of system variables including minimum or base storage temperature, storage capacity, and geographic location. The method relies on heavily pre-processed site-specific radiation and weather data which is used with system properties to predict the quantities necessary for correlation. The method yields long-term monthly and annual performance predictions which are so accurate that they can serve simultaneously for preliminary design, economic optimization, and final design, eliminating the need for simulation.     

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 studies of the position of the auxiliary heater in thermosyphon solar water heating systems

This paper investigates the effect of the physical location of the auxiliary source of energy in thermosyphon solar water heaters and shows that the performance of the system can be optimised with respect to the geometry of the system components. The investigation has been based on a domestic thermosyphon solar water heating system, which was simulated using the TRNSYS programme. The annual solar fraction of the system, at the weather and socioeconomic conditions of Cyprus, is, at best, approximately 77% with an in-tank auxiliary heater configuration and 86% with an external auxiliary heater. It is demonstrated that the arrangement with the external auxiliary unit has a higher collector efficiency and results in a higher annual solar fraction. In the case of in-tank auxiliary, the system performance increases with the height of the auxiliary position from the bottom of the storage tank; with the auxiliary at the bottom of the storage tank the annual solar fraction is approximately 59%, compared to 77% when the auxiliary is located at the top of the tank. The system performance also depends on the height of the collector return from the bottom of the tank.     

泰安地区利用太阳能采暖的技术经济分析

从泰安地区气候条件、可采用的采暖系统形式、集热器选择及面积计算、蓄热水箱容量计算等几个方面对太阳能采暖系统的技术性分析,又通过对几种方案的年计算费用的比较对系统的经济性分析,论证了太阳能采暖在泰安地区是切实可行的。为泰安地区今后在建筑中推广利用太阳能采暖工作提供参考。