Energy savings for solar heating systems

In this paper the realistic behaviour and efficiency of heating systems were analysed, based on long term monitoring projects. Based on the measurements a boiler model used to calculate the boiler efficiency on a monthly basis was evaluated. Comparisons of measured and calculated fuel consumptions showed a good degree of similarity. With the boiler model, various simulations of solar domestic hot water heating systems were done for different hot water demands and collector sizes. The result shows that the potential of fuel reduction can be much higher than the solar gain of the solar thermal system. For some conditions the fuel reduction can be up to the double of the solar gain due to a strong increase of the system efficiency. As the monitored boilers were not older than 3 years, it can be assumed that the saving potential with older boilers could be even higher than calculated in this paper.     

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.     

基于气象数据的家用太阳能热水系统利用优化方法研究

太阳能热水系统每天的太阳能制热水量及用户在不同季节天气中的热水需求量会不同,但家用太阳能热水系统的水箱容量都普遍偏大,在冬季或阴雨天使用辅助能源加热的整箱热水有剩余,造成辅助能源的浪费。提出依据天气状况利用历年气象数据预测太阳能制热水量的方法,从而自动控制水箱水量来提高家用太阳能热水系统热利用效率。在自动控制器中录入各地区历年太阳辐射量数据库和各种太阳能热水系统的太阳能制热水量计算程序,用户可依据当天天气预报的天气状况使用自动控制器就能预测出当天的太阳辐射量可能加热的热水量,再根据自身的热水需求情况选择是否增加辅助能源热水量。此方法在满足用户使用需求的同时提高太阳能有效利用率,也避免浪费不必要的辅助能源。研究以太阳能资源较贫乏地区(Ⅳ区)的南宁市为例进行实验验证,结果可预测出当天太阳辐射制热水量,并能够控制热水出水温度与目标水温的偏差在5℃以内。     

Energy flow through the flat-plate solar water heating system

A simulation model of the flat-plate solar water heating system is used to trace the flow of energy through the system from the collector to storage and load for a system operating under idealised sunlight conditions.     

Design and performance of large low-cost solar water heating systems

An analysis is presented for the monthly performance evaluation of a simple design low cost solar water heating systems. A sample of typical results is presented which confirms their suitability as solar heating systems for summer peacking or as solar preheaters for year around loads.     

Net energy analysis of domestic solar water heating installations in operation

The potential of solar water heating systems to reduce domestic energy use is frequently acknowledged. However there are two factors that are rarely discussed when studying this technology. Firstly the real performance of the installed systems in operation, and secondly a life cycle perspective of its energy use. These two issues are reviewed in this paper, and a field study in Ireland is also presented. In the review, some studies show that measured real performance of domestic solar water heating systems can be lower than expectations. Concerning their life cycle energy performance, existing studies show that the initial energy investment for the systems (their embodied energy) is a small portion of the energy savings over their lifetime with calculation paybacks generally lower than 2 years. On the field study carried in Ireland, representative of a maritime north European climate, the ‘energy payback’ based on the expected energy savings is between 1.2 and 3.5 years, values comparable to previous studies considering the less favourable climate and installation characteristics. However the measured energy savings generally worsened the life cycle energy performance of this technology and thus increased the energy payback period. The study concludes that while there is a real potential for life cycle energy savings through domestic solar water heating installations, devising mechanisms to ensure proper design, installation and operation of systems is essential for this technology.     

A numerical model for optimal receiver array and mass flow rate in residential solar water heating systems

In this paper, the performance of a pressurised evacuated tube solar collector system using internal heat pipes is presented. The system was optimised for the seasonal supply of hot service water for residential use in Pretoria, South Africa. The prediction of seasonal hourly performance trends along with the maximum thermal performance at the optimal receiver array and manifold mass flow rate was of major concern in this investigation. A mathematical model representing the thermal performance of the system was developed and numerically implemented in Engineering Equation Solver. The dynamic performance of collectors with 15, 20 and 25 tubes was determined throughout all the seasons. Moreover, the performance of the residential solar water heating system with a 20-tube collector was investigated in detail for mass flow rates of 0.03, 0.05 and 0.07?kg/s.     

Reduction of carbon dioxide emissions by solar water heating systems and passive technologies in social housing

Growing global concern regarding climate change motivates technological studies to minimize environmental impacts. In this context, solar water heating (SWH) systems are notably prominent in Brazil, primarily because of the abundance of solar energy in the country. However, SWH designs have not always been perfectly developed. In most projects, the installation option of the solar system only considers the electric power economy aspects and not the particular characteristics of each climatic zone. Thus, the primary objective of this paper is to assess the potential of carbon dioxide reduction with the use of SWH in comparison with electric showers in social housing in several Brazilian climatic zones. The Brazilian government authorities have created public policies to encourage the use of these technologies primarily among the low-income population. The results of this paper indicate that hot climactic regions demonstrate a low reduction of CO₂ emissions with SWH installations. Thus, solar radiation is not useful for water heating in those regions, but it does lead to a large fraction of household cooling loads, implying a demand for electrical energy for air conditioning or requiring the adoption of passive techniques to maintain indoor temperatures below threshold values.     

Transient analysis of parallel plate forced circulation solar water heating system

In this paper, a transient analysis of a forced circulation solar water heating system with and without heat exchangers in the collector loop and storage tank has been presented for a parallel flat plate collector. The effect of various water heating system parameters on its performance have been studied. Numerical calculations have been made for a typical cold day viz. 26 January 1980 in Delhi.     

Feasibility analysis of domestic solar water heating systems in Greece

The excessive usage of fossil fuels has world-widely caused chain environmental consequences. An interesting solution to this problem is the systematic exploitation of available renewable energy sources, including solar energy. Greece is located in a major geographical region with an abundant and reliable supply of solar energy, even during the winter. In as much, one cannot disregard the significant dependency of the country on imported fuels, since almost 70% of its domestic energy consumption is covered by oil and natural gas imports. Despite the relative local sun abundance, during the last 10 years the local solar collectors market illustrates a sluggish behaviour, in comparison with the impressive numbers of sales during the 1980–1990 decade. At a first glance, such an occurrence characterizes a controversy. In an attempt to find a rational explanation of this peculiar situation, an integrated cost-benefit analysis is carried out taking into consideration the vast majority of the parameters affecting solar thermal energy production cost. The resulting numerical values are then compared with the corresponding ones coming from alternative hot-water production techniques. Accordingly, a quite extensive sensitivity analysis is carried out, in order to demonstrate the impact of the main techno-economic parameters on the fiscal behaviour of contemporary solar hot water production systems. The results obtained not only explain with sufficient accuracy the current local market situation but also demonstrate the specific actions that if realized they may boost solar collector sales in the corresponding local market.     

Evaluation of retrofitting gas-fired cooling and heating systems into BCHP using design optimization

The influence of energy prices on the feasibility of a retrofit is investigated. The retrofit describes the conversion of a system from HVAC to BCHP for energy-saving. This includes two optimal retrofit design models, of which the exergetic efficiency and annual costs (AC) are the separate objective functions. The retrofit scheme is planned to insert gas engines as prime movers into the original system, which have adopted gas-fired absorption chillers. The solutions of the optimizations show that such a retrofit can result in a remarkable rise in exergetic efficiency but is not viable with current energy prices. The contradictory solutions reveal a gap between the current energy prices system of the country and the present energy situation. Further investigation gives the critical lines of which each divides the coordinate plane of natural gas–electric prices into two parts of benefit and deficit. If the electric price rises to a certain extent, the retrofit will be advantageous both in benefit and energy-saving. So it is really an urgent task to reform the energy prices system in China. Conclusions may be helpful for other similar retrofit projects, and for legislators and the government which are responsible for improving the energy market in China.     

The impacts of solar water heating in low-income households on the distribution utility’s active, reactive and apparent power demands

In Brazilian low-income households, water-heating requirements are typically met by electrical showerheads. On average, 73.1% of all residential units in the country are equipped with these resistance-heating devices, with nominal powers ranging from 3 to 8 kW. This situation imposes a considerable burden on the electricity utility companies, since electrical showerheads typically represent the highest load but the lowest utilization (load factor) in a residential consumer unit. Furthermore, typical utilization times coincide with, and contribute to, the electrical power demand peaks in Brazil, rendering these low-cost, high-power electrical devices a high-cost consumer for the electrical system to cater for. For low-income residential consumers, electricity tariffs are subsidized, and utilities must therefore make a considerable investment in infrastructure for a limited return. In this paper we analyze the impacts of solar water heating in low-income households on the distribution utility active, reactive and apparent power demands. We have monitored a statistically representative group of low-income residences equipped with a compact domestic solar water heater in Florianopolis - Brazil for 1 year. We show that in comparison with identical residential units using electrical showerheads, with the adoption of solar water heating the reductions in the active, reactive and apparent power demands on the distribution utility were 49%, 29% and 49% respectively.     

Economic evaluation and optimization of solar systems for space and domestic water heating

Solar energy will be utilized only if it proves to be economical. In this paper, we evaluate the economic feasibility of the use of solar energy for space and domestic water heating systems for a house in Benghazi, Libya. A comprehensive evaluation considerering 324 cases representing the proper ranges of economic and load factors and based on the annual equivalent cost approach is given. The optimum collector area for each case is determined. The results obtained and relationships developed permit generalizations that can be applicable in other locations.     

A general chart for sizing the collectors of solar heating systems

A general chart is developed for calculating the collector area required to provide a prescribed value of the annual load fraction for solar heating systems. The relationship between collector area and annual load fraction can then be easily obtained for specified collector design parameters, load and location. The construction of the chart is based on correlating data generated by the f-chart method. Data of 13 locations in the U.S.A. are considered. The good agreement of the results obtained by the present simple method and the f-chart for both space and combined space/domestic water heating proves that the two methods are almost of the same accuracy. Since the present chart is not location dependent and allows direct comparison of different collectors, it is a very valuable design aid for sizing and selecting solar collectors.     

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.     

Modelling and analysis of the dynamic behavior of a forced circulation solar water heating system with storage tanks in series

A generalized model for a forced circulation solar water heating system with storage tanks in series is presented in which the loss of heat through an insulation lagging is considered, and the periodic time variation of the intensity of solar radiation, as well as both the ambient air temperature and the temperature of cold water entering the first main tank, is taken into account. Using the Laplace transformation, an exact solution is presented which, under certain conditions, reduces to an approximate solution. The conditions for convergence to the approximate solution are discussed, and figures are presented comparing it with the exact solution for several different sets of conditions. In this communication, the effect of the number of storage tanks on the outlet temperature of the hot water and the effect of various water heating system parameters on its performance have been analytically investigated. Numerical calculations have been made for a typical cold day.     

A simplified approach for evaluating performance of a solar water heater with side losses

We present a simplified method which could be used to calculate, to a first approximation, the energy losses from the sides of a solar collector/water heater. The inclusion of energy losses through the side walls of a reasonable thickness reduce the efficiency of the system by about 5%. The integrated side energy losses for the whole day are not equal for the various segments of the wall. Hence, it will be economical to construct wedge shape walls, thickness decreasing downward, instead of usual rectangular walls.     

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

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

Outdoor test method to determine the thermal behavior of solar domestic water heating systems

The dynamics of the market, the generation of new promotion programs, fiscal incentives and many other factors are to be considered for the massive application of solar domestic water heating systems (SDWHS) mainly of the compact thermosiphon type, makes it necessary to choose simple and inexpensive procedure tests that permit to know their characteristic thermal behaviors without an official standard being necessary. Moreover, it allows the comparison among systems and offers enough and reliable information to consumers and manufacturers.In most developing countries, an official national standard for SDWHS is not available, therefore it is necessary to adopt an international test procedure in which the cost and time of implementation is very important. In this work, a simple and inexpensive test method to determine the thermal behavior of SDWHS is proposed.Even though these procedure tests do not have an official standard structure they permit, by comparing different solar systems under identical solar, ambient, and initial conditions, the experimental determination of: (a) the maximum available volume of water for solar heating; (b) water temperature increment and available thermal energy at the end of the day; (c) temperature profiles (stratification) and the average temperature in the storage tank after it is homogenized; (d) the average global thermal efficiency; (e) water temperature decrement and energy lost overnight; and (f) the relationship between hot water volume and solar collector area as function of the average heating temperature. An additional proposed test permits to know the heat losses caused by the reverse flow in the collector loop. These tests will be carried out independently of the configuration between the solar collector and the storage tank, the way the fluid circulates and the type of thermal exchange. The results of this procedure test can be very useful, firstly, for the local solar manufacturers’ equipment in order to design and optimize its products comparing their systems against a reference system under identical test conditions and secondly, by the consumers in order to select the most suitable system. The resulting experimental data for a particular thermosiphon system is presented and discussed.     

Experimental study of the burning-cave hot water soil heating system in solar greenhouse

In the northern China areas, the traditional heating methods are widely used in solar greenhouse, for example: electric heating, hot air heating, hot water heating, burning-cave heating etc. If copying the assuring building indoor environment of constant heating ways into solar greenhouse, it will further increase building energy consumption, thus improving the efficiency of energy utilization, establishing appropriate growing environment, and realizing the agricultural waste recycling are important ways of consistent with the Chinese conditions, construction of sustainable development, improving the efficiency of the greenhouse production. To solve the problem of traditional heating method for high heating energy consumption, the inharmonious between greenhouse air temperature and soil temperature, uneven soil temperature, the research build the burning cave hot water soil heating system of solar greenhouse experimental platform in accordance with principle of energy cascade utilization. This experiment platform will transfer burning cave internal heat into soil heating system. The soil is evenly heated by system. Through testing the actual operation effect of the burning cave hot water soil heating system of new solar greenhouse, electric heating system, no taking any heating measures system, burning cave hot water soil heating system of solar greenhouse can improve the soil average temperature 5 ∼ 6 °C. This research provides experimental basis for practical applications and promotion.