Technical and economic performance of residential solar water heating in the United States

This paper examines the regional, technical, and economic performance of residential rooftop solar water heating (SWH) technology in the U.S. It focuses on the application of SWH to consumers in the U.S. currently using electricity for water heating, which currently uses over 120 billion kWh per year. The variation in electrical energy savings due to water heating use, inlet water temperature and solar resource is estimated and applied to determine the regional “break-even” cost of SWH where the life-cycle cost of SWH is equal the life-cycle energy savings. For a typical residential consumer, a SWH system will reduce water heating energy demand by 50–85%, or a savings of 1600–2600 kWh per year. For the largest 1000 electric utilities serving residential customers in the United States as of 2008, this corresponds to an annual electric bill savings range of about $100 to over $300, reflecting the large range in residential electricity prices. This range in electricity prices, along with a variety of incentives programs corresponds to a break-even cost of SWH in the United States varying by more than a factor of five (from less than $2250/system to over $10,000/system excluding Hawaii and Alaska), despite a much smaller variation in the amount of energy saved by the systems (a factor of approximately one and a half). We also consider the relationships between collector area and technical performance, SWH price and solar fraction (percent of daily energy requirements supplied by the SWH system) and examine the key drivers behind break-even costs.     

Solar water heating potential in South Africa in dynamic energy market conditions

This paper is an attempt to determine the potential for solar water heating (SWH) in South Africa and the prospects for its implementation between 2010 and 2030. It outlines the energy market conditions, the energy requirements related to residential and commercial water heating in the country and the solar water heating market dynamics and challenges. It was estimated that 98% of the potential is in the residential sector and the rest in the commercial sector. The total thermal demand for 20 years for water heating was estimated to 2.2 EJ. A ‘Moderate SWH implementation’ will provide 0.83 EJ of clean energy until 2030 and estimated cost savings of 231 billion rand. For an ‘Accelerated SWH implementation’ these figures are 1.3 EJ and 369 billion rand. The estimated accumulated reduction of CO₂ emissions due to SWH can be as high as 297 Mt. The increased affordability of residential hot water due to SWH is an important social factor and solar water heating has a strong social effect.     

Potential application of solar thermal systems for hot water production in Hong Kong

This paper presents the evaluation results of conventional solar water heater (SWH) systems and solar assisted heat pump (SAHP) systems for hot water production in Hong Kong. An economic comparison and global warming impact analysis are conducted among the two kinds of solar thermal systems and traditional water heating systems (i.e. electric water heaters and towngas water heaters). The economic comparison results show that solar thermal systems have greater economic benefits than traditional water heating systems. In addition, conventional SWH systems are comparable with the SAHP systems when solar fractions are above 50%. Besides, analysis on the sensitivity of the total equivalent warming impact (TEWI) indicates that the towngas boosted SWH system has the greatest potential in greenhouse gas emission reduction with various solar collector areas and the electricity boosted SWH system has the comparative TEWI with the SAHP systems if its solar fraction is above 50%. As for SAHP systems, the solar assisted air source heat pump (SA-ASHP) system has the least global warming impact. Based on all investigation results, suggestions are given on the selection of solar thermal systems for applications in Hong Kong.     

Optimizing 100%‐renewable grids through shifting residential water‐heater load

A low‐carbon electricity supply for Australia was simulated, and the installed capacity of the electrical grid was optimized by shifting the electricity demand of residential electric water heaters (EWHs). The load‐shifting potential of Australia was estimated for each hour of the simulation period using a nationwide aggregate EWH load model on a 90 × 110 raster grid. The electricity demand of water heaters was shifted from periods of low renewable resource and high demand to periods of high renewable resource and low demand, enabling us to effectively reduce the installed capacity requirements of a 100%‐renewable electricity grid. It was found that by shifting the EWH load by just 1 hour, the electricity demand of Australia could be met using purely renewable electricity at an installed capacity of 145 GW with a capacity factor of 30%, an electricity spillage of 20%, and a generation cost of 15.2 ¢/kWh. A breakdown of the primary energy sources used in our scenario is as follows: 43% wind, 29% concentrated solar thermal power, and 20% utility photovoltaic. Sensitivity analysis suggested that further reduction in installed capacity is possible by increasing the load‐shifting duration as well as the volume and insulation level of the EWH tank.     

A dynamic linear programming approach to market allocation of renewable resources in the italian energy system: The case of solar thermal and biogas technologies

In Italy solar thermal energy and energy from biogas are two possible means of reducing dependence on energy imports. Using a multiperiod LP model (MARKAL) the authors assessed the likely potential of both technologies under various circumstances. The study covered the period 1980–2005, in five segments of five years. It focused only on the subsystem of the energy end-uses which can be substituted for by solar thermal and biogas technologies. The overall non-renewable sources which can be saved in 20 years by these technologies total 450 PJ (1 PJ = 10^{1 5} J) if the fuel prices rise at 0 per cent average annual, 1450 PJ if the fuel prices rise at 4.2 per cent average annual, 1860 PJ if the fuel prices rise at 7.2 per cent average annual and 3780 PJ if the fuel prices rise at 15 per cent average annual. However the most competitive technologies appear to be solar water heaters used mainly in the private and commercial sectors and biogas systems used mainly in the agricultural sector. The study was carried out by APRE under ENEA (formerly CNEN) contract and was intended to serve as an analytical basis for establishing an overall development and demonstration strategy for end-use renewable technologies in Italy.     

Effective solar radiation based benefit and cost analyses for solar water heater development in Taiwan

To reduce greenhouse gases emissions, promoting solar water heaters (SWHs) has become an essential national policy in Taiwan. To implement this policy effectively, the applicability of SWHs in different regions must be analyzed. Previous studies generally performed SWH benefit-cost analyses based on total annual solar radiation; however, this method may overestimate energy production benefits because, for an SWH, the solar energy captured today cannot be preserved. Therefore, this study proposes the concept of effective solar radiation (ESR), which is based on potential heat output estimated using tap water temperature and solar radiation in each region. The benefits of SWHs are then assessed based on the number of effective days and ESR, instead of using total annual solar radiation. A procedure is established to evaluate the applicability of SWHs in each region based on proposed benefit-cost analyses. Possible outcomes of a national SWH program are estimated. The sensitivities of essential factors, including collector efficiency, installation cost, and discount rate, are also analyzed. Analytical results show that the ratios of ESR to total annual solar radiation for regions in Taiwan are about 82-89%. The payback periods vary at 6-15 years for different regions and heater types being replaced. The national program is expected to reduce greenhouse gases emissions by approximately 150,000 tons eCO₂ annually.     

Potential for solar water heating in Zimbabwe

This paper discusses the economic, social and environmental benefits from using solar water heating (SWH) in Zimbabwe. By comparing different water heating technology usage in three sectors over a 25-year period, the potential of SWH is demonstrated in alleviating energy and economic problems that energy-importing countries like Zimbabwe are facing. SWH would reduce coincident electricity winter peak demand by 13% and reduce final energy demand by 27%, assuming a 50% penetration rate of SWH potential demand. Up to $250 million can be saved and CO₂ emissions can be reduced by 29% over the 25-year period. Benefits are also present at individual consumer level, for the electricity utility, as well as for society at large. In the case of Zimbabwe, policy strategies that can support renewable energy technologies are already in current government policy, but this political will need to be translated into enhanced practical activities. A multi-stakeholder approach appears to be the best approach to promoting widespread dissemination of SWH technologies.     

Prediction of greenhouse gas reduction potential in Japanese residential sector by residential energy end-use model

A model is developed that simulates nationwide energy consumption of the residential sector by considering the diversity of household and building types. Since this model can simulate the energy consumption for each household and building category by dynamic energy use based on the schedule of the occupants’ activities and a heating and cooling load calculation model, various kinds of energy-saving policies can be evaluated with considerable accuracy. In addition, the average energy efficiency of major electric appliances used in the residential sector and the percentages of housing insulation levels of existing houses is predicted by the “stock transition model.” In this paper, energy consumption and CO₂ emissions in the Japanese residential sector until 2025 are predicted. For example, as a business – as-usual (BAU) case, CO₂ emissions will be reduced by 7% from the 1990 level. Also evaluated are mitigation measures such as the energy efficiency standard for home electric appliances, thermal insulation code, reduction of standby power, high-efficiency water heaters, energy-efficient behavior of occupants, and dissemination of photovoltaic panels.     

Subsidy programs on diffusion of solar water heaters: Taiwan’s experience

Financial incentives are essentially one of the key factors influencing diffusion of solar water heaters in many countries. Two subsidy programs were initiated by the government of Taiwan in 1986 (1986–1991) and 2000 (2000–present), respectively. Those long-term national programs are considered to be the driving force on local market expansion. In 2008, the regional subsidy programs for solar water heaters were announced by Kaohsiung city and Kiemen county, which resulted in the growth in sales. A revised subsidy was also initiated by the government of Taiwan in 2009. The subsidy is 50% more. However, the tremendous enlargement of market size with a high-level ratio of subsidy over total installation cost might result in a negative impact on a sustainable SWH industry and long-term development of the local market, which is associated with system design and post-installation service. This paper aims to address the relative efficiency and pitfalls of those national and regional programs.     

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.     

Collector cum storage solar water heaters with and without transparent insulation material

The integrated collector–storage solar water heaters are less expensive and can offer the best alternative for domestic applications particularly to small families to meet hot water requirements. The top heat losses of such solar water heaters are quite high during the night and the temperature of stored hot water is considerably reduced unless covered with extra insulating cover in the evening which is a cumbersome job. The transparent insulation material widely used in Europe for space heating can also minimize top heat losses, if used in such solar water heaters. For this purpose, two units of solar collector cum storage water heaters have been designed to study the relative effect of TI for retaining solar heated hot water for a night duration. Both units were identical in all respects except one of them was covered with TIM. The theoretical exercise was carried out to evaluate design parameters of ISC which revealed total heat loss factor (U_L) 1.03 W/m² K with TIM glazed against 7.06 W/m² K with glass glazed. The TIM glazed has been found to be quite effective as compared to glass glazed SWH and yielded hot water at higher temperature by 8.5 to 9.5°C the next morning. The storage efficiency of such solar water heaters has been found to be 39.8% with TIM glazed as compared to 15.1% without TIM. The TIM glazing means not having to cover the ISC solar water heater with a separate insulator cover in the evening and thus makes its operation much simpler.     

应用层次分析法综合评价家用太阳热水器

采用层次分析法对家用太阳热水器的热性能，经济性，可靠性以及使用寿命等多项因素进行综合评价。通过建立评价家用太阳热水器的层次结构及确定评价指标，借助计算机计算模型，可简捷，方便地排出各种不同型式家用太阳热水器的优劣，为用户选择产品和生产厂家优化设计提供依据，也可用来作为评比太阳热水器的一种方法。     

Challenges in the implementation of energy conservation

Penetration of energy-conservation devices during the last few years has led to progressive reductions in estimated total energy demands for the U.S. and other countries. Relatively rapid reductions have been implemented in areas dominated by social habits. Significant gains are noted in the transportation and space-conditioning sectors.Following innovative marketing, a number of passive (solar) retrofit procedures has been found to yield effective potentials for significant energy-conservation gains in the residential sector. Examples are ceiling insulation to reduce energy requirements for space-conditioning, fenestration devices for windows to reduce air-conditioning loads during hot summer months, and insulation retrofits on inadequately insulated water heaters.Over the long term, the major challenge in the residential sector concerns the design, construction, and marketing of energy-efficient homes. These will utilize far less energy, on the average, than is currently needed, without requiring major changes in the lifestyles of inhabitants. Systems studies allowing for technological innovation (e.g. solar water heating and solar-space conditioning for multiple-family aggregates, walls with thermal storage, use of improved materials and designs) and removal of obstacles (e.g. zoning restrictions, outdated building codes, difficulties for proper financing) present outstanding challenges to designers, municipal planners, and politicians alike.Innovation in industrial production is more clearly defined than in the residential sector and will be facilitated by (a) educating consumers on the importance of life-cycle costing in making intelligent choices in the capital-goods market and (b) by marginal pricing of energy supplies. Optimized economic evaluations from the consumer's and manufacturer's points of view are currently widely introduced and should play important roles in defining the long-term acceptabilities of more energy-efficient appliances, vehicles, and other products. In many cases (e.g. cogeneration, district heating), market penetration will be delayed because of existing excess capacity (as in the electric utility sector) or current availability of alternative techniques that will remain less costly than better new technologies when life-cycle costing is properly done. Delays in the introduction of improved technologies will generally be minimized by allowing fuel-price escalation to recover marginal costs.Inventions and innovations in technologies that are not motivated by energy-conservation goals (e.g. introduction of new generations of motor vehicles, aircraft, entertainment facilities such as color TVs, stereo systems, and video-tapes) have played dominant roles in improving what many people view as their standard of living. These inventions and innovations have been responsible for rapidly escalating energy-use rates. In developing future scenarios for energy conservation, it is important to allow room for important inventions not yet conceived or not yet available (e.g. personal electric vehicles) and for the corresponding radical changes in energyuse patterns that may accompany their introduction. Without this provision, the most benign energy-conservation scenarios may not justifiably be viewed as maintaining or augmenting the quality of life.The concluding remarks briefly address life-cycle costing and the relations between per capita income and energy use in different countries.     

Energy conservation options for residential water heaters

Current designs of gas-fired and electric water heaters present a substantial opportunity for energy conservation through reductions in jacket losses, pilot losses and flue-gas losses. A systematic analysis and comparison of alternative energy-conserving designs has been carried out. Promising options for gas-fired water heaters include increased insulation, thermostat setback, forced-draft burners with intermittent ignition and flue closure, and instantaneous (low storage) designs. High efficiency electric water heaters incorporate increased insulation, thermostat setback, solar preheat, and heat pump operation. The evolution and market penetration of these alternative designs, some of which are commercially available at this writing, can reduce the energy usage for residential water heating in the U.S. by approx. 0.3 quads in the near-term, and up to 1.0 quads in the long-term as advanced designs achieve widespread saturation.     

Optimization of the design of polygeneration systems for the residential sector under different self-consumption regulations

Polygeneration systems enable natural resources to be exploited efficiently, decreasing CO₂ emissions and achieving economic savings relative to the conventional separate production. However, their economic feasibility depends on the legal framework. Preliminary design of polygeneration systems for the residential sector based on the last Spanish self-consumption regulations RD 900/2015 and RD 244/2019 was carried out in Zaragoza, Spain. Both regulations were applied to individual and collective installations. Several technologies, appropriate for the energy supply to residential buildings, for example, photovoltaics, wind turbines, solar thermal collectors, microcogeneration engines, heat pump, gas boiler, absorption chiller, and thermal and electric energy storage were considered candidate technologies for the polygeneration system. A mixed integer linear programming model was developed to minimize the total annual cost of polygeneration systems. Scenarios with and without electricity sale were considered. CO₂ emissions were also calculated to estimate the environmental impact. Results show that RD 900/2015 discourages the investment in self-consumption systems whereas the RD 244/2019 encourages them, especially in renewable energy technologies. Moreover, in economic terms, it is more profitable to invest in collective self-consumption installations over individual installations. However, this does not necessarily represent a significant reduction of CO₂ emissions with respect to individual installations since the natural gas consumption tends to increase as its unit price decreases because of the increase of its consumption level. Thus, more appropriate pricing of natural gas in residential sector, in which its cost would not be reduced when increasing its consumption, would be required to achieve significant CO₂ emissions reduction. In all cases, the photovoltaic panels (PV) are competitive and profitable without subsidies in self-consumption schemes and the reversible heat pump (HP) played an important role for the CO₂ emissions reduction. In a horizon to achieve zero CO₂ emissions, the net metering scheme could be an interesting and profitable alternative to be considered.     

Outlook for solar water heaters in Taiwan

The share of indigenous energy supply continuously decreases over the last two decades in Taiwan. The development and use of renewable energy sources and technologies are becoming vital for the management of energy supply and demand. For promotion of solar water heaters, the incentive programs were firstly initiated in the period of 1986–1991 and re-initiated from 2000 to the present. These programs create an economic incentive for the end users and have a drastic effect on the popularization of solar water heaters. To further promote solar water heaters during the current incentive program period, several key factors are addressed. In addition to the cost of solar water heaters and energy price index, the potential market of solar water heaters in Taiwan is associated with the climatic conditions, population structure, urbanization, building type of housing and status of new construction.     

Energy savings versus costs of implementation for demand side management strategies within an energy-efficient tropical residence

As renewable energy sources and net-zero energy homes become increasingly pervasive within the residential building industry, further reductions in consumption patterns will occur through demand side management (DSM). DSM can include measures such as energy-efficient system design, automated control and energy management systems, or policies and monitoring systems intended to alter user behavior. For an energy-efficient modern residence designed within a tropical context, several DSM strategies are considered for reductions in operational-phase energy consumption: a lightweight, thermally high-performing building envelope, installation of light dimmers to enhance user control of lighting, and comparison of a solar hot water system versus a point-of-use electric water heater to produce hot water for bathing demands. The energy-consumption savings associated with the three DSM strategies are simulated and normalized to an energy savings per cost of implementation basis in kWh per 1000 Thai Baht (THB) for comparison. The results show that financial investments in low-energy hot water heaters (i.e., solar water heating systems) result in relatively higher energy savings per unit financial investment than the other two strategies. Conversely, the installation of a lightweight, well-insulated envelope is highly expensive relative to its associated energy savings over a 25-year time frame. The savings associated with the insulated envelope, light dimmers, and hot water production strategies are evaluated at 80, 609 and 657 kWh/1000 THB investment, respectively.     

Solar water heaters as a pre-heating system for industrial processes

Although solar heating for hot-water production is a mature technology, the general public’s knowledge is mostly confined to domestic uses such as cleaning and washing. In the industrial sector, there is a demand for thermal energy of lower than 250 °C, but to develop applications for industrial heat processes, real energy savings are one of the key issues determining customers’ choice of energy. However, the solar gain from solar water heaters based on the lab tests may be misleading, so field measurements are important to evaluate a system’s financial viability. This study presents 1 year of field measurements for solar water heaters installed as the pre-heating system in a poultry slaughterhouse. The system efficiency is compared with that installed in a dormitory, where the hot-water consumption pattern is similar to that in the domestic sector. These results demonstrate that the amount of global solar radiation and hot-water consumption patterns have a significant effect on the system’s efficiency. A simple cost-to-benefit analysis is also presented based on this real application for solar water heaters.     

Diffusion of solar water heaters in regional China: Economic feasibility and policy effectiveness evaluation

Whereas the technical feasibility of solar water heaters (SWHs) has long been established, the economic feasibility of SWHs in regional China remains to be examined. This paper constructs cost models to calculate costs per unit energy saving of SWHs in 27 Chinese provincial capital cities. The cost effectiveness of SWHs is examined at the national level. At a micro level, we analyze the financial attractiveness of consumers’ investment in SWHs. A panel data model is employed to evaluate the effectiveness of a subsidy program in rural China. The results show that SWH costs, ranging from 0.305 to 0.744 CNY/kW h, are much lower than those of other major renewable energies across China. This finding indicates that the diffusion of SWHs is a cost-effective way to reach China’s renewable energy target. For consumers, incentive programs for SWHs are needed to improve the financial attractiveness of the devices in China. Existing subsidy policies for rural China have failed to significantly enhance the deployment of SWHs. The causes of the failure are examined and a new incentive program is suggested for rural areas of the country.     

Comparative studies on thermal performance of water-in-glass evacuated tube solar water heaters with different collector tilt-angles

To performance comparative studies, two sets of water-in-glass evacuated tube solar water heater (SWH, in short) were constructed and tested. Both SWHs were identical in all aspects but had different collector tilt-angle from the horizon with the one inclined at 22° (SWH-22) and the other at 46° (SWH-46). Experimental results revealed that the collector tilt-angle of SWHs had no significant influence on the heat removal from solar tubes to the water storage tank, both systems had almost the same daily solar thermal conversion efficiency but different daily solar and heat gains, and climatic conditions had a negligible effect on the daily thermal efficiency of systems due to less heat loss of the collector to the ambient air. These findings indicated that, to maximize the annual heat gain of such solar water heaters, the collector should be inclined at a tilt-angle for maximizing its annual collection of solar radiation. Experiments also showed that, for the SWH-22, the cold water from the storage tank circulated down to the sealed end of tubes along the lower wall of tubes and then returned to the storage tank along the upper wall of solar tubes with a clear water circulation loop; whereas for the SWH-46, the situation in the morning was the same as the SWH-22, but in the afternoon, the cold water from the storage tank on the way to the sealed end was partially or fully mixed with the hot water returning to the storage tank without a clear water circulation loop, furthermore, such mixing became more intense with the increase in the inlet water temperature of solar tubes. This indicated that increasing the collector tilt-angle of SWHs had no positive effect on the thermosiphon circulation of the water inside tubes. No noticeable inactive region near the sealed end of solar tubes for both systems was observed in experiments.