Simulation of a solar domestic water heating system using a time marching model

This paper presents the modelling and simulation of a solar water heating system using a time marching model. The results of simulations performed on an annual basis for a solar system, constructed and operated in Yugoslavia, which provides domestic hot water for a four-person family are presented. The solar water heater consists of a flat-plate solar collector, a water-storage tank, an electric heater, and a water-mixing device. The mathematical model is used to evaluate the annual variation of the solar fraction with respect to the volume of the storage tank, demand hot water temperature required, difference of this temperature and preset storage tank water temperature, and consumption profile of the domestic hot water demand. The results of this investigation may be used to design a solar collector system, and to operate already designed systems, effectively. The results for a number of designs with different storage tank volumes indicate that the systems with greater volume yield higher solar fraction values. The results additionally indicate that the solar fraction of the system increases with lower hot water demand temperature and higher differences between the mean storage water and the demand temperatures. However, when a larger storage tank volume is used, the solar fraction is less sensitive to a variation of these operation parameters.     

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.     

Effects of auxiliary heater on annual performance of thermosyphon solar water heater simulated under variable operating conditions

A thermosyphon solar water heating system with electric auxiliary heater was simulated using the TRNSYS simulation program. Location of the auxiliary heater, inside the storage tank or connected in series between the system and the user, was studied using the TMY meteorological data for Los Angeles, California. Simulations were performed for two different water load temperatures (60 and 80°C) and for two types of daily hot water volumes (250 and 150 l). Four types of daily hot water consumption profiles were used in the present study, namely; the widely used Rand profile, continuous, evening and morning profiles. Also, the simulation is extended to cover the effects of thermal and optical properties of the flatplate collector and the volume of the storage tank. The results show that if water is drawn on a schedule corresponding to the Rand draw profile, the system operates with higher efficiency when the auxiliary heater is located in the storage tank than when the auxiliary heater is outside the storage tank. When operated with each of the other three draw schedules, however, better performance is achieved by locating the auxiliary heater outside the tank. The increase in solar fraction depends on the load profile and volume, temperature setting, as well as the quality of the collector and the storage tank volume. When the values of the parameters F_R(τα)_n and F_RU_L are changed from 0.8 and 16 kJ/h m²°C to 0.6 and 30 kJ/h m²°C, the solar fraction decreases by approximately 40–50%.     

太阳能地面采暖系统蓄热水箱容积分析

通过分析太阳能采暖系统所需蓄热鼍与建筑热负荷、太阳能集热量日变化规律之间的关系,得出太阳能采暖系统所需蓄热水箱容积的理论算式.根据拉萨、银川、西宁、西安等地的太阳辐射强度及建筑热负荷的日变化规律,模拟得出系统所需蓄热量变化规律;并对各种蓄热温差下对应的蓄热水箱容积进行了模拟分析,结果表明:太阳能采暖系统所需蓄热量随太阳集热器的集热量与建筑热负荷之间的差值增大而增加;蓄热水箱容积随蓄热温差增大而减小,当蓄热水温达到80℃时,在各种地面采暖系统取水温度下,单位集热器面积所需蓄热水箱容积趋于相等.     

Modelling and simulation of a solar absorption cooling system

A detailed numerical simulation model is developed for a commercially available solar absorption chiller. The model incorporates the performance data of a Yazaki-manufactured water-cooled system. We take into consideration the variation of the COP and cooling water temperature. Using a summer season's meteorological data for an arid location in the Sahara desert, the system performance is computed for different collector types, areas and storage volumes. The results show that an optimum storage volume/collector area ratio exists. Also a high solar fraction can be obtained with relatively small areas of collectors, even when the collectors are of the inexpensive type. The interesting feature was that the system operated at design load conditions with generator temperatures as low as 80°C owing to the fact that very low cooling water temperatures are available in the dry conditions of the Sahara. The study establishes the high potential of solar operated, water-cooled absorption coolers especially for arid conditions.     

Performance and economics of annual storage solar heating systems

Annual storage is used with active solar heating systems to permit storage of summer-time solar heat for winter use. This paper presents the results of a comprehensive computer simulation study of the performance of active solar heating systems with long-term hot water storage. A unique feature of this study is the investigation of systems used to supply backup heat to passive solar and energy-conserving buildings, as well as to meet standard heating and hot water loads.

Findings show that system performance increases linearly as storage volume increases, up to the point where the storage tank is large enough to store all heat collected in summer. This point, the point of “unconstrained operation”, is the likely economic optimum. In contrast to diurnal storage systems, systems with annual storage show only slightly diminishing returns as system size increases. Annual storage systems providing nearly 100% solar space heat may cost the same or less per unit heat delivered as a 50 per cent diurnal solar system. Also in contrast to diurnal systems, annual storage systems perform efficiently in meeting the load of a passive or energy-efficient building. A breakeven cost 4¢–10¢/kWh is estimated for optimal 100 per cent solar heating in the U.S.A.     

A parametric design study of solar ponds

The salt stratified solar pond is found to be a reliable solar collector and storage system. This paper discusses the effect of varying certain design parameters on pond steady-state temperatures. These significant parameters are sizing parameters—pond surface area and depth of the pond; operating parameters—storage volume and the heat extraction fraction; and geo-climatic parameters3s?olar radiation, water table depth and upper convective zone thickness. Studies indicate that there is an optimum depth and storage volume of the pond for each application in terms of temperature and heat load desired.     

Thermal stratification in small solar domestic storage tanks caused by draw-offs

Storage tanks with different cold water inlet devices for small Solar Domestic Hot Water (SDHW) systems are compared. The objective of the investigation is to reveal the impact of the cold water inlet device on the thermal stratification in two marketed tanks and to evaluate the possible enhancement in the annual system performance of small solar heating systems. Two different marketed inlet designs are compared, one connected to a small curved plate placed above the inlet tube, the other one connected to a much larger flat plate. The cold domestic water enters the stores in vertical direction from the bottom of the tanks. Temperature measurements were carried out for different operating conditions. It was shown that the thermal stratification inside the two tanks depends differently on the flow rate, the draw-off volume, as well as the initial temperature in the storage tank. To carry out system simulations, a multi-node storage model was used and expanded by an additional input variable to model the mixing behaviour depending on the operating conditions. The inlet device with a comparatively large plate compared to the less favourable design results in an increase of the solar fraction of about 1–3%-points in annual system simulations with a solar fraction of about 60% and fairly large domestic hot water flow rates. This corresponds to a reduction of the auxiliary energy supply of the solar heating system of about 3–7% (58–155 MJ/year) for the investigated solar domestic hot water system.     

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.     

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.     

Effect of obstacles with different opening means on thermal stratification in hot water storage tanks

为研究具有内置隔板的太阳能蓄热水箱隔板开孔尺寸及位置对其内部热分层效果的影响,对9种隔板开孔位置的太阳能蓄热水箱内温度场进行了数值分析,结果显示：在相同的流动参数及开孔面积条件下,隔板中心开1个圆孔的水箱热分层效果最好。对于多开孔的水箱,开孔位置对水箱内热分层影响不大,但对蓄热量影响显著。对于隔板中心开1个圆孔的水箱,在不同流动参数条件下,冷、热水出口温差随着冷水入口流速的增大呈先增后减的趋势,当冷水入口流速大于0.9 m/s时,减弱了热分层的稳定性。     

Performance calculations for closed-loop air-to-water solar hybrid heating systems with and without a rock bed in the solar air heater

A transient analysis has been carried out on a hybrid solar water heater which comprises a rock bed air heater with optimum design parameters in conjunction with an air-to-water transverse fin shell-and-tube heat exchanger (mixed air and unmixed water type) in which cold water from the storage tank receives heat from the hot air coming out of the air heater which flows in the shell at right angles to the water flowing in the tubes. The system's performance has been evaluated for typical winter weather conditions in Delhi for different combinations of flow rates of air and water for different volumes of the water storage tank. No hot water is assumed to be withdrawn from the tank to serve the load. A comparative analysis of the system's performance with and without a rock bed in the air heater reveals about 11°C higher temperature of storage tank in the former at 50 kg/h m² air flow rate. With both the air heater types, although the system performance was observed to increase with the rates of air and water flow, no significant improvement in system performance was achieved at .     

A simplified method for optimal design of solar water heating systems based on life-cycle energy analysis

Significant energy mismatch exists in solar water heating systems as the time and amount of solar energy supply are usually different from that of hot water demand. Using a hot water storage tank can reduce or eliminate such mismatch in short term while it is difficult to avoid this mismatch in long term. In many optimal design and life-cycle analysis methods, the energy mismatch is ignored which causes the system performance to be overestimated and also misleads the optimal design of the system. This paper presents a simplified method for optimizing the key parameters of solar water heating systems based on life-cycle energy analysis. This optimal method considering the energy mismatch phenomenon can be implemented through two steps. In the first step, a simplified energy model based hourly energy matching different components of the system, is developed for determining the operating performance of system with different solar collector areas and water storage volumes. In the second step, the law of diminishing marginal utility is employed to determine the optimum size of the system. The optimum size is identified when the maximal life-cycle net energy saving is achieved. A case study on the application of the proposed method in a building is presented as well.     

Modeling of the CSU heating/cooling system

This paper resents a thermal simulation of the Colorado State University solar house. A computer model of the solar energy system was developed and computer runs were made using one year of meteorological data to determine the important design features. The system consists of a flat plate solar collector, main storage tank, service hot water storage tank, auxiliary heater, absorption air conditioner with cooling tower and heat exchangers between the collector and storage, storage and service hot water tank and storage and residence. This system very closely models the CSU house in operating mode one.The results are in the form of monthly integrated values for the pertinent energy quantities. In addition, results are presented which show the effect on the system performance of the collector tilt, collector area and number of covers.     

Parametric study of the overall performance of a solar hot water system

The effect of varying individual parameters on the overall performance and feasibility of a solar hot water system has been investigated by a mathematic model which employs the concept of a decay constant and feasibility factor. TRNSYS results were used as inputs to apply the model. Parameters investigated include emittance of the collector plate, collector plate absorptance, transmittance absorptance product, storage tank set temperature, mass flow rate, load, and ratio of tank volume to collector area. It was found that the model is a useful tool for solar system design. Specific examples are detailed for illustration purposes.     

Study on performance of solar assisted air source heat pump systems for hot water production in Hong Kong

This paper reports the investigation results on application of the solar assisted air source heat pump systems for hot water production in Hong Kong. A mathematical model of the system is developed to predict its operating performance under specified weather conditions. The optimum flow rate from the load water tank to the condenser is proposed considering both the appropriate outlet water temperature and system performance. The effect of various parameters, including circulation flow rate, solar collector area, tilt angle of solar collector array and initial water temperature in the preheating solar tank is investigated, and the results show that the system performance is governed strongly by the change of circulation flow rate, solar collector area and initial water temperature in the preheating solar tank.     

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.     

Economical and environmental assessment of an optimized solar cooling system for a medium-sized benchmark office building in Los Angeles,California

This paper presents a systematic energetic, economical, and environmental assessment on a solar cooling system for a medium-sized office building in Los Angeles, California by means of system modeling. The studied solar cooling system primarily consists of evacuated tube solar collectors, a hot water storage tank, a single-effect LiBr–H₂O absorption chiller, and a gas-fired auxiliary heater. System performance optimization and sensitivity analysis were conducted by varying two major parameters (i.e. storage tank volume and collector area). The results suggest that a trade-off exists between economic performance indicated by the equivalent uniform annual cost (EUAC) and the energetic/environmental performance indicated by the solar fraction and CO₂ reduction percentage, respectively. The cost of carbon footprint reduction was defined and served as an indicator for the overall system performance. Based on this indicator, the optimal system design could be found for a solar cooling system. The approach adapted in this study can be applied to other buildings located in different climate zones to reveal the cost and benefits of solar cooling technologies and facilitate decision-making.     

Optimal design for a thermosyphon solar water heater

Through the use of TRNSYS, a transient simulation program, optimization of design parameters for a thermosyphon solar water heater was studied for two regions in Jordan represented by two cities, namely Amman and Aqaba. The optimum value of a parameter is defined as the value which maximizes the annual solar fraction of a system. This paper includes a good deal of information concerning sizing of common components of thermosyphon solar water heaters operated under certain condition (load volume, distribution profile and temperature) using weather data of Jordan. The results indicate that the solar fraction of the system can be improved by 10–25% when a proper choice is used for each studied parameter. It is also shown that the solar fraction of a system installed in Aqaba (hot climate) is less sensitive to some parameters than the solar fraction of a similar installed in Amman (mild climate).     

Parametric Analysis of Solar Heating and Cooling Systems for Residential Applications

Abstract

In this paper, a parametric analysis of two solar heating and cooling systems, one using an absorption heat pump and the other one using an adsorption heat pump, was performed. The systems under investigation were designed to satisfy the energy requirements of a residential building for space heating/cooling purposes and domestic hot water production. The system with the absorption heat pump was analyzed upon varying (i) the solar collectors’ area, (ii) the volume of the hot water storage, (iii) the volume of the cold water tank, and (iv) the climatic conditions. The system with the adsorption heat pump was evaluated upon varying (i) the inlet temperature of hot water supplied to the adsorption heat pump, (ii) the volume of the hot water storage, (iii) the volume of the cold water tank, and (iv) the climatic conditions. The analyses were performed using the dynamic simulation software TRNSYS in terms of primary energy consumption, global carbon dioxide equivalent emissions, and operating costs. The performance of the solar heating and cooling systems was compared with those associated with a conventional system from energy, environmental and economic points of views in order to evaluate the potential benefits.