Performance of solar assisted heatpump systems in residential applications

In this experimental study, several solar-assisted heating and cooling configurations have beenconsidered for a basic system comprised of a two-speed heat pump, photovoltaic (PV) arrays, solar thermal collectors, and thermal storage. The objective of the study was to determine the performance of the PV arrays at decreased insolation, the effects of air preheat by solar thermal energy on heat pump operation, and cooling system performance under two different configurations. During the entire operation, the PV arrays converted 4.7 per cent (9.5 MWh) of the incident solar insolation to d.c. power, of which 54.6 per cent was used by the residence. This contributed 23.4 per cent of the total house electrical demand. The remaining 45.4 per cent of the output was fed to the utility, indicating the arrays and the heat pump were not properly sized with each other. Based on results from the winter heating operation, it is shown that for the particular heating system consdered, the best performance is attained when the solar heating is used alone. By using the heat pump as a booster, the remaining available solar energy left in the storage tank can be used with good seasonal performance factor. Summer cooling operation consisted of two sequential cooling configurations. In the first cooling test, the heat pump was operated to either the house or storage when the PV array generation level was greater than the energy demand of the heat pump and associated equipment. When the array output level was less than the cooling system demand, the operating strategy was that of an off-peak cooling operation to chill the water storage. Utilization of chilled water storage was not realized in the first cooling test because of the inherent inefficient design of the Tri-X coil. The capacity at low-speed heat pump operation was too small to effect significant cooling of the water loop; whereas high-speed heat pump operation in attempting to chill water (fan operation absent) caused frosting of the coil. The heat pump was utilized only to maintain chilled water storage in the second cooling test, without heat transfer through the Tri-X coil. Cooling system performance obtained in cooling test 2 using the Ametex exchanger was considerably improved over the test 2 performance with the Tri-X coil.     

Preliminary performance of CSU Solar House I heating and cooling system

The NSF/CSU Solar House I solar heating and cooling system became operational on 1 July 1974. During the first months of operation the emphasis was placed on adjustment, “tuning”, and fault correction in the solar collection and the solar/fuel/cooling subsystems. Following this initial check out period, analysis and testing of the system utilizing a full year of data was begun. This paper discusses the preliminary performance of the heating and cooling system.

During the period 1 August 1974–31 January 1975, approximately 40 per cent of the cooling load was provided by solar energy. Solar heating over the same period of time provided 86 per cent of the space heating load and 68 per cent of the domestic hot water heating load. These percentages represent a total solar contribution of 33,996 MJ delivered to load (8061 MJ to the cooling unit; 20,687 MJ to heating; 5248 MJ to hot water). Natural gas accounted for 22,442 MJ, total. In addition, preliminary analysis has provided several significant results associated with the operating characteristics of the solar system and the individual components.     

Computerized design and economic evaluation of an aqua-ammonia solar operated absorption system

A computerized simulation and design of solar operated NH₃---H₂O absorption refrigeration cycles is presented. The program receives input data and calculates the building, cooling and heating loads initially. Next the absorption cycle is designed along with the solar collectors and auxiliaries. Various economic parameters are also calculated from which the most favorable system may be selected. Two examples were run: one for the Knesset building in Jerusalem (Israeli parliament) and the other for an American office building. Results indicate the existence of various mininal operating parameters, e.g. collectors' outlet temperature, etc. For the Knesset building some 46 per cent of the annual heating and cooling demand may be provided by the solar system. At the 1979 rate of energy cost increase in Israel the system payback period was estimated at 9 yr with a present value total saving of $42.5 m. Yet higher values were obtained for the American office building at about 81 per cent solar fraction for the compound cooling and heating system.     

Solar energy and the residence-some systems aspects

The feasibility, in an energy flow sense, of providing heating, cooling and electrical power for individual homes using some form of solar energy converter on the roof of each residence is considered. A model for home power requirements and solar insolation which reflects residence construction, local weather and geographic location is developed. This is used to demonstrate that 50–90 per cent of the homes in the U.S.A. could be self-powered from solar energy providing sufficient insulation is used and adequate energy conversion techniques are developed.     

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 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.     

Solar gasification of coal, activated carbon, coke and coal and biomass mixtures

Subbituminous coal, activated carbon, coke and a mixture of coal and biomass were gasified using direct solar irradiation in a 23-kW solar furnace located at the U.S. Army White Sands Missile Range, White Sands, New Mexico. The sunlight was focused directly on the coal (or alternate fuel) bed being gasified through a window in the reactor. Steam or CO₂ (in different experiments) was passed through the solar-heated coal bed where it reacted with the coal and thus formed a combustible product gas that contained the energy content of both the coal and the sunlight. More than 40 per cent of the sunlight arriving at the focus external to the reactor was chemically stored as fuel value in the product gas. Since there were considerable solar losses because of the reflectivity of the window and the window aperture being smaller than the focal-spot size, it is estimated that in excess of 60 per cent of the solar energy that entered the reactor was chemically stored. The product-gas production rate increased with increased solar power, and when steam was used for gasification, the product-gas composition and thus heating value were almost independent of solar power. A typical moisture-free gas composition was 54 per cent H₂, 25 per cent CO, 16 per cent CO₂, 4 per cent CH₄ and 1 per cent higher hydrocarbons. Activated carbon and a uniform mixture of coal and biomass were also gasified with similar efficiencies but slightly different product-gas compositions. Coke showed a lower solar conversion efficiency. Solar gasification offers several advantages over conventional oxygen-blown gasifiers: (1) commercial grade oxygen is not required, (2) almost twice as much gas per ton of coal can be achieved because no coal is burned to provide process heat and because the gas contains energy from both coal and the sun, and (3) the system has very low thermal inertia and is insensitive to thermal shock, making it very adaptable to rapidly changing solar conditions such as passing clouds.     

联合太阳能沼气的冷热电供能系统的集成及经济性分析

冷热电联供是一种先进、高效的能源系统,目前在我国应用的主要问题是天然气成本高,导致系统经济性差。太阳能和沼气是非常清洁的可再生能源,在我国来源广泛且廉价。将冷热电联供系统与太阳能、沼气完美地结合起来,集成为联合太阳能沼气的冷热电供能系统。该系统较为合理的组合方式是采用太阳能沼气池作为燃料提供装置,采用微型燃气轮机、余热锅炉、溴化锂吸收式制冷机、蒸汽换热器等作为供电、供冷和供热机组,采用太阳能集热器、换热器等装置为沼气池加热,太阳能不足时采用尾气加热。该系统能够实现能量的梯级利用,提高一次能源利用率,达到综合用能的目的,同时可有效治理环境。以某酒店作为该系统的用户对象,分析其经济性并与常规模式进行对比。结果表明,该系统一次能源利用率为74.8%,而常规模式为62.3%;综合能源价格为0.3398元/(k W·h),而现阶段电网电价约为0.6元/(k W·h);环境与减排评价指标也具有明显优势。     

Experience with fully operational solar-driven 10-ton LiBr/H2O single-effect absorption cooling system in Thailand

A solar-driven 10-ton LiBr/H₂O single-effect absorption cooling system has been designed and installed at the School of Renewable Energy Technology (SERT), Phitsanulok, Thailand. Construction took place in 2005, after which this system became fully operational and has been supplying cooling for our main testing building's air-conditioning. Data on the system's operation were collected during 2006 and analyzed to find the extent to which solar energy replaced conventional energy sources. Here, we present these data and show that the 72 m² evacuated tube solar collector delivered a yearly average solar fraction of 81%, while the remaining 19% of thermal energy required by the chiller was supplied by a LPG-fired backup heating unit. We also show that the economics of this cooling system are dominated by the initial cost of the solar collector array and the absorption chiller, which are significantly higher than that of a similar-size conventional VCC system.     

The effects of operation parameter on the performance of a solar-powered adsorption chiller

A solar-powered adsorption chiller with heat and mass recovery cycle was designed and constructed. It consists of a solar water heating unit, a silica gel-water adsorption chiller, a cooling tower and a fan coil unit. The adsorption chiller includes two identical adsorption units and a second stage evaporator with methanol working fluid. The effects of operation parameter on system performance were tested successfully. Test results indicated that the COP (coefficient of performance) and cooling power of the solar-powered adsorption chiller could be improved greatly by optimizing the key operation parameters, such as solar hot water temperature, heating/cooling time, mass recovery time, and chilled water temperature. Under the climatic conditions of daily solar radiation being about 16–21 MJ/m², this solar-powered adsorption chiller can produce a cooling capacity about 66–90 W per m² collector area, its daily solar cooling COP is about 0.1–0.13.     

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.     

Solar-assisted absorption heat pumps feasibility

An absorption system can be used for space cooling as well as for space heating. This dual purpose may be achieved by using the system as heat pump in wintertime. Absorption heat pump heating may be an interesting alternative, particularly for countries where there is a shortage of electric power.When an absorption unit is used as heat pump, its mode of operation is not modified: the internal temperatures of the cycle are only raised. Commercially available LiBr units were tested as heat pumps. COP and heating capacity were considered as a function of cold source temperature for different temperatures of the useful heat. The COP arrived at 1.7, which must be considered a high value for a thermally driven heat pump.Simulations were carried out in order to compare the performance of “conventional” solar, solar assisted heat pump and the combined series system under two different climate conditions. The series system showed performance 25–75 per cent better than “conventional” solar alone.     

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.     

Studies of the direct input of solar energy to a fossil-fueled central station steam power plant

Seven possible methods of absorbing solar energy as direct thermal input to an 800 MW, fossil-fueled, central station steam power plant have been studied. Irrespective of method, the solar heat is first collected by an array of flat mirrors and concentrated on a tower-mounted absorber where it is transferred into the power cycle. The heat absorbing methods studied were heating of feed-water, evaporation of water, superheating of steam, combined evaporation and superheating, reheating of steam, air preheating, and combined air preheating, and feedwater heating. Factors considered were relative capital cost, energy conversion efficiency and complexity of design, operation and control. Combined evaporation and superheating proved to be the preferred method because of its high utilization of solar energy, relatively low indicated capital cost and only moderate complexity in design, operation and control. Feedwater heating also has very desirable capital cost, design and operating aspects, but suffers from the drawback that over 30 per cent of the solar energy absorbed is, in effect, lost because of degradation of the steam cycle efficiency.     

Thermal management of hydrogen refuelling station housing on an annual level

A housing insulation of hydrogen refuelling station is vital from the aspect of safe operation of equipment in an environment that is installed. To secure hydrogen supply during the whole year, this work brings the solution for both cooling and heating insulation equipment inside of hydrogen refuelling station installed in Croatia, Europe. This hydrogen refuelling station was designed as an autonomous photovoltaic-hydrogen system. In the interest of improving its energy efficiency, an optimal thermal management strategy was proposed. To select the best technological solution for thermal management design which will maintain optimal temperature range inside the housing in cold and warm months, a detailed analysis of the system components thermodynamic parameters was performed. Optimal operating temperatures were established to be 25 °C in summer and 16 °C in winter, considering components working specifications. Insulation, type of cooling units, and heaters have been selected according to the HRN EN 12831 and VDI 2078 standards, while the regime of the heating and cooling system has been selected based on the station's indoor air temperature. The annual required heating and cooling energy were calculated according to HRN EN ISO 13790 standard, amounting to 1135.55 kW h and 1219.55 kW h, respectively. Annual energy share obtained from solar power plant used for the heating and cooling system resulted in 5%. The calculated thermal management system load turned out to be 1.437 kW.     

Performance analysis of a solar-powered/fuel-assisted Rankine cycle with a novel 30 hp turbine

The subject of this analysis is a novel hybrid steam Rankine cycle, which was designed to drive a conventional open-compressor chiller, but is equally applicable to power generation. Steam is to be generated by the use of solar energy collected at about 100°C, and is then to be superheated to about 600°C in a fossil-fuel fired superheater. The steam is to drive a novel counter-rotating turbine, and most of its exhaust heat is regenerated. A comprehensive computer program developed to analyze the operation and performance of the basic power cycle is described. Each component was defined by a separate subroutine which computes its realistic off-design performance from basic principles. Detailed predicted performance maps of the turbine and the basic power cycle were obtained as a function of turbine speed, inlet pressure, inlet temperature, condensing temperature, steam mass flow rate, and the superheater's fuel consumption rate. Some of the major conclusions are: (1) the turbine's efficiency is quite constant, varying in the range of 68.5–76.5 per cent (75 per cent at design) for all conditions, (2) the efficiency of the basic power cycle is 18.3 per cent at design, more than double as compared to organic fluid cycles operating at similar solar input temperatures, at the expense of adding only 20 per cent non-solar energy. This, combined with the fact that actual organic Rankine cycles operate typically at temperatures above 140°C, predicts that this system would be economically superior by using less than half of the collector area and by also using less expensive collectors.     

Total planning of combined district heating,cooling and power generation systems for a new town—Part II

The objective of this study is to introduce one of the main results of the project for studying energy conservation technologies in a new airport town, which is organized by the Osaka Science and Technology Center, Japan. First, based on the estimated energy demands in the new town, technological aspects are investigated for the district heating, cooling and hot water supply system. Then, the economic and energy saving characteristics are compared for several alternative systems according to the differences of the type of absorption refrigerating machine and so forth. Assuming that a combined heat and power plant is used as the heat source plant of the district thermal distribution system, the optimal combined district heating, cooling and power generation system has been selected from a comprehensive economic viewpoint. Lastly, it is ascertained that if fuel costs continue to rise at the rate of 8 per cent per year, the best energy conservation system becomes superior economically to the conventional district thermal distribution system.     

The ITW solar heating system: an oldtimer fully in action

Public awareness of energy in the early 1970s stimulated a number of projects on alternative ways of heating. The ‘Institut für Thermodynamik und Wärmetechnik’ (ITW) of the University of Stuttgart has been operating a solar heating system since 1985. Ever since, this system has been minutely monitored. In particular, the storage was painstakingly considered as it was intended to serve as a pilot facility for the much discussed problem of seasonal storage. This storage unit should be simple and cheap but heavily instrumented in order to obtain many and accurate data and it should be versatile in order to gain knowledge for operation. The solar heating is provided by collectors that are unglazed, so a heat pump is required for appropriate heating temperatures. However, the heat pump allowed for a combination of heating and cooling in our system and this proved to be very advantageous, as cooling energy is more expensive and more in demand in our building than heating energy.

The system was used in various seasonal cycles with changing conditions. It has now been operating for almost 15 years. During this period, neither storage nor collectors caused any trouble. Some difficulties were experienced with the heat pump. The first one had to be replaced; we made suggestions for improvement of the second one. This provided good COPs but there has been an occasional defect. The experience with our solar heating system was so satisfactory that, based on the knowledge gained from it, large housing projects in Friedrichshafen and Hamburg and an office building project in Chemnitz were conceived and built and are now being monitored.     

Energy resource requirements of a solar heating system

The methods of energy analysis have been applied to a liquid-based, short-term storage solar space and water heating system suitable for a single family dwelling in Toronto. This system, which in many respects represents a worst case for solar heating, takes 1.0–3.5 years of operation to conserve the energy resources required to build, operate and maintain the system. Alternatively, over the twenty year lifetime of the system, the energy resources used indirectly by the solar heating system amount to between 6 and 24% of the direct energy resources conserved by the system. These considerations do not significantly alter the energy-conservation characteristics of the solar heating system unless thermally-generated electricity is used as backup for a 50% solar heating system which replaces oil or gas heating; in this case, only 4–9% of the energy resources are conserved by the solar system. A factor of three variation in energy resource use in collector materials was found in a sample of 7 flat-plate collectors with steel-based collectors using the least. The total energy embodied in the collector was about double that found in the materials alone. The collectors and annual operating energy for the pumps were found to be the two most significant factors in the analysis. An appendix summarizes the energy resource requirements embodied in the materials used for collectors.     

Thermal analysis of three zone solar pond

This paper presents a periodic analysis of a three zone solar pond as a solar energy collector and long term storage system. We explicitly take into account the convective heat and mass flux through the pond surface and evaluate the temperature and heat fluxes at various levels in the pond during its year round operation by solving the time dependent Fourier heat conduction equation with internal heat generation resulting from the absorption of solar radiation in the pond water. Eventually, an expression, for the transient rate at which heat can be retrieved from the solar pond to keep the temperature of the zone of heat extraction as constant, is derived. Heat retrieval efficiencies of 40.0 per cent, 32.1 per cent, 28.3 per cent and 25.5 per cent are predicted at collection temperatures of 40, 60, 80 and 100°C, respectively. the retrieved heat flux exhibits a phase difference of about 30 to 45 days with the incident solar flux; the load levelling in the retrieved heat flux improves as the thickness of the non-convective zone increases. the efficiency of the solar pond system for conversion of solar energy into mechanical work is also studied. This efficiency is found to increase with collection temperature and it tends to level around 5 per cent at collection temperatures about 90°C.