Solar Domestic Hot Water Research Articles

太陽熱利用の普及拡大に向けた環境価値活用制度の検討

This paper investigated and proposed a new running-cost subsidy system utilizing the environmental value of solar thermal in order to promote expansion of solar thermal use. The program is not a conventional subsidy program for the initial cost but a new program of providing a subsidy for running a system while confirming user status. It is for the national or local government to provide incentives in accordance with the environmental value based on the amount of solar thermal use. The similar scheme to the proposed system, there is Green Thermal Certificate system in Japan, Australia REC system and UK RHI system, and we discussed the common points and differences between the proposed system and these similar systems. Feature of the proposed system is, by the conversion factor that defines for each measurement method is to convert the amount of solar thermal use to the environmental value amount, and the user can select how to measure. It was verified as NEDO research project for the accuracy of the measurement method. This project has adopted specified measuring instruments as reference and conducted experimental study using actual or artificial loads at residential houses and condominiums in a total of 101 fields across the country. The study set out the target values of error within ±10% for built-in simple calorimeters and external simple calorimeters and less than ±20% for deeming measurement. The study has confirmed that both built-in and external simple calorimeters give high precision in measurement: the average errors of the annual solar heat consumption volumes stood at -1.0% and +2.6%, respectively. As for deeming measurement, the annual solar heat consumption volume has been calculated with the EESLISM simulation program. The calculation has used as parameters HASP meteorological data in the regions with apparatus installed, characteristic values of equipment, expected hot water supply loads and other elements. As a result, the average error of the annual solar heat consumption volumes was -12.2%, which has also satisfied the targeted error range. The subsidy per MJ for the standard cost scenario of a domestic solar hot water system (modified 3.0 to 3.5 people household, solar collector area: 4m2, and hot water tank: 200L) is 5.50 yen. It is based on the national average amount of solar thermal use of simulation calculation (5,647MJ/annual) under the condition of hot water demand M2 (400L/daily hot water supply) at 20 sample places in Japan. The amount of subsidy by measurement method is approx. 310,000 yen, 280,000 yen, 260,000 yen and 220,000 yen when it is measured with a specified measuring instrument, external simple calorimeter, built-in simple calorimeter and deeming measurement, respectively. The subsidy per MJ under a similar UK RHI system is 6.35 yen and its amount is approx. 400,000 yen (four-people household, 4m2 of solar collector area, and 250L of hot water tank) and thus the proposed system is not difficult even slight difference in the estimation conditions is taken into consideration. The basic national energy plan has a goal of requiring new houses and buildings to comply with energy-efficiency standards in stages. Solar thermal utilization is essential to realize the government goal and the program will contribute to its realization as one of measures.

Optimization of the PCM-integrated solar domestic hot water system under different thermal stratification conditions

Many researchers have investigated how to increase the overall efficiency of solar-driven thermal systems. Several key parameters, such as collector efficiency and storage tank characteristics, may impose some constraints on the annual solar fraction (ASF) of such systems. In this paper, the behaviour of integrating the phase change material (PCM) in SDHW systems is modelled and optimized numerically. Coupled collector and partly stratified PCM-embedded storage tank governing equations are utilized to simulate the overall performance of the system. The developed code presents the monthly behaviour of the system including the solar fraction and the storage tank temperature profile. The results indicate that the stratification of the storage tank will increase the ASF up to about 4.6%. Additionally, it is found that the optimum amount for the PCM and its melting temperature is changed as the tank stratification goes from the fully mixed to the fully stratified state. Integrating the PCM in the storage tank leads to increases of 5.3% in the ASF for a single-node tank, while a rise of only 0.7% is seen for the stratified storage tank.

Retrofitted Solar Domestic Hot Water Systems for Swedish Single-Family Houses—Evaluation of a Prototype and Life-Cycle Cost Analysis

According to recent technology road maps, system cost reductions and development of standardised plug-and-function systems are some of the most important goals for solar heating technology development. Retrofitting hot water boilers in single-family houses when installing solar collectors has the potential to significantly reduce both material and installation costs. Previous studies have investigated such retrofitting, using theoretical simulations and laboratory tests, but no actual installations were made and tested in practice. This article describes the installation, measured performance and cost effectiveness of a retrofitting solution that converts existing domestic hot water heaters to a solar domestic hot water system. The measured performance is characterised by the monthly and annual solar fractions. The cost effectiveness is evaluated by a life-cycle cost analysis, comparing the retrofitted system to a conventional solar domestic hot water system and the case without any solar heating system. Measurements showed that approximately 50% of the 5000 kWh/year of domestic hot water consumption was saved by the retrofitted system in south Sweden. Such savings are in agreement with previous estimations and are comparable to the energy savings when using a conventional solar domestic hot water system. The life-cycle cost analysis showed that, according to the assumptions and given climate, the return on investment of the retrofitted system is approximately 17 years, while a conventional system does not reach profitability during its lifetime of 25 years.

Performance of SDHW systems with fully mixed and stratified tank operation under radiative regimes with different degree of stability

In this paper we investigate whether, and to what extent, the radiative regime has an influence on the performance of a solar domestic hot water system. We compared the results of two mathematical models (a more accurate model with stratification in the storage tank and a simple, less accurate, fully mixed model), simulating the dynamic behaviour of a system with two flat-plate solar collectors, under three commonly used tapping cycles. Four summer and four winter days with different radiative regimes have been selected for simulations. For the less stable winter day of 19 January (relative sunshine σ = 0.4–0.7), when the system operates with a constant working fluid flow rate of 0.01 kg/(s m2) and hot water is extracted according to a medium consumption tapping cycle, the mean value predicted by the more accurate, stratification model, for the useful heat gain by the solar collector is 388.87 W. Using this value as a reference, the MBE and RMSE values of the results predicted by the less accurate model are 41.54 W and 293.28 W, respectively. This proves that approximation models may not be used successfully for simulation. For variable working fluid mass flow rate, depending on the level of incoming solar irradiation and collecting surface area, the simplified model accuracy increases with the increase of the radiative regime stability.

Thermal efficiency of a solar power system in a collective residential structure based on performance tests

Here, we evaluated the efficiency of a solar domestic hot water system in a dormitory in Rzeszow. Radiance exposure was analyzed based on information obtained from an online database (the meteorological station in Rzeszow-Jasionka). The mean seasonal and annual solar irradiation of a flat surface and surfaces inclined at 30°, 45°, and 60° from the horizontal plane in Rzeszow were determined. The mean monthly irradiation was compared with that in selected European cities based on the Photovoltaic Geographical Information System meteorological database. The thermal solar panel area and final consumption of solar energy in Poland from 2005 to 2014 are also presented. The characteristics of the analyzed building, distribution of the inhabitants, and consumption of hot water from 2009 to 2014 are described. The heat demand was determined by hot water consumption per day per person based on hot water consumption metering and heat metering. The solar energy conversion efficiency was determined after the solar thermal collectors were installed in the building. The relation of the heat demand calculated based on the thermal energy utilized to heat water per person and heat metering for heating water was also analyzed. We also present a method of calculating the annual (stratified by month) ratio of solar energy utilization. Energy savings for the building were calculated. Improvements that could further increase the solar energy efficiency and shorten the investment payback period are proposed.

Investigation and modelling of the centralized solar domestic hot water system in residential buildings

The solar domestic hot water (DHW) system is applied as a building energy saving technique. The centralized system, which is considered to provide both energy efficiency by exploiting solar energy and high-level services to the customers as the occupants could have access to the hot water at any time during the day, is widely used among the various forms of solar DHW systems. The real performance of a centralized solar DHW system is measured, based on which a model describing the solar energy collection, water supply, and terminal water use is built. The simulation reveals that the actual energy performance of the centralized DHW system is closely related with the occupant behaviour, i.e., the water use patterns of the occupants residing in a building. This study concluded that whether the DHW system is energy efficient or not could not be evaluated without considering the water use patterns of the occupants.

The performance of an evacuated tube solar hot water system in a domestic house throughout a year in a northern maritime climate (Dublin)

The performance of an evacuated tube solar hot water system, installed on a domestic house (occupied by 2 adults and 3 children) in Dublin, Ireland has been intensely monitored over the period of a year. Instrumentation to measure flows and temperature were installed throughout both the solar circuit and domestic hot water system, in addition to solar radiometers being installed on site to measure both global and diffuse radiation. The overall annual solar system efficiency at the installation angle of 29° to the horizontal was 63%. Heat losses from all aspects of the solar circuit and hot water system have also been quantified. The ongoing hot water usage pattern of the house occupants over the course of the year has been analysed as well as the changing attitudes to the use of hot water since the installation of the new solar system. This has shown some evidence that more hot water tends to be used on days where there has been more direct sunlight. Net present value calculations based on 5years of operation however, have shown that the retrofit of such a system is not currently economically viable, although it has produced a carbon saving of approximately 1.1tonnes CO2eq over its first 5years of operation.

Mixed integer linear programming for the design of solar thermal energy systems with short-term storage

In this paper a mixed integer linear programming (MILP) model is developed to facilitate the design of a solar domestic hot water system. The MILP model does so by optimising the area of roof mounted flat plate solar thermal collectors and the volume of thermal energy storage that are required to minimise the annual capital and operational cost of the overall energy system. A new formulation of the MILP is presented in which the model is decomposed into two sub-modules that are iteratively solved. One module determines the optimal sizing of the solar thermal collectors and storage tank. The other module optimises the hourly energy production of the solar collectors based on the impact that the collector inlet stream temperature has on the collector efficiency. This iterative formulation is compared to both a conventional MILP formulation and a dynamic simulation model to determine if the iterative formulation results in a design tool that is able to accurately represent the dynamic behaviour of a solar domestic hot water system, while maintaining the computational tractability and scalability of the model. The novel iterative MILP formulation is applied to a case study of an apartment building in Rheinfelden, Switzerland. The results indicate that conventional MILP formulations overestimate the amount of energy that solar thermal collectors are able to produce, resulting in an oversizing of the required thermal storage tank. However, the iterative MILP model more accurately predicts the energy production of the solar collector, leading to the more realistic sizing of the component technologies. This result is important when evaluating the financial feasibility of solar thermal collector systems in comparison to other renewable generation technologies, as an overestimation of the generation potential of solar thermal collectors may result in an incorrect assessment of their investment attractiveness.

Classifications of central solar domestic hot water systems

Currently, there are many means by which to classify solar domestic hot water systems, which are often categorized according to their scope of supply, solar collector positions, and type of heat storage tank. However, the lack of systematic and scientific classification as well as the general disregard of the thermal performance of the auxiliary heat source is important to DHW systems. Thus, the primary focus of this paper is to determine a classification system for solar domestic hot water systems based on the positions of the solar collector and auxiliary heating device, both respectively and in combination. Field-testing data regarding many central solar DHW systems demonstrates that the position of the auxiliary heat source clearly reflects the operational energy consumption. The consumption of collective auxiliary heating hot water system is much higher than individual auxiliary heating hot water system. In addition, costs are significantly reduced by the separation of the heat storage tank and the auxiliary heating device.

Analysis of solar thermal systems and future development possibilities in Lithuania

Solar thermal systems with a total solar panel area varying from 2 to 204 m2 have been installed in Lithuania for over 20 years. The reviewed solar thermal domestic hot water systems in Lithuania produce up to 528 kWh per year per one square meter of solar collector absorber area. However, the performance of these systems varies depending on the type of energy users, equipment and design of the systems, as well as their maintenance. The aim of this paper was to analyse solar thermal systems from the perspective of energy production and economic benefit as well as to outline the differences of their actual performance compared to the numerical simulation results. A number of different solar thermal systems in Lithuania were selected for the analysis varying both in equipment used (flat type solar collectors, evacuated tube collectors) and type of energy user (domestic hot water heating, swimming pool, district heating). The simulation software Polysun 8.1 was used for evaluation of solar thermal system performance, as well as financial analysis of alternatives of these systems. The results of the analysis showed that in the analysed cases the gap between measured and modelled data of heat energy produced by solar thermal systems was up to approximately 11%. The calculation of internal rate of return showed that a grant is required in most cases for solar thermal projects to be fully profitable.

The United States regulatory compact and energy poverty

Utility regulation in the United States (US) was founded partly on a consensus that raw marketplace economics ignored social justice, including universal service goals. The century-old ‘regulatory compact’ in most jurisdictions offers ‘just and reasonable rates’ in exchange for investment in public services. Justice has come to justify such low-income supports as discounted rates, arrearage forgiveness, limitations on service termination, and low/no cost energy efficiency. The consensus for regulation has now evolved to encompass carbon reduction, and has led to, amongst other things, the promotion of domestic forms of renewable energy known as ‘distributed generation’ (DG). However, such technologies potentially threaten the current regulatory balance that includes ameliorating energy poverty, because DG reduces utility sales but not utility fixed costs and so contributes to higher bills for low-income households that cannot afford such DG investments as rooftop solar, solar domestic hot water, and cogeneration.The aim of this paper is to analyze how utility regulation might evolve to encompass modern energy developments, thus addressing both the goals of reducing carbon and amerliorating fuel poverty. It begins by reviewing the origin of US utility regulation and describes the regulatory compact that resulted. It then discusses possible balancing measures, including tax-based subsidies, system benefit charges (taxes) on DG, stricter application of just and reasonable regulatory principles, and low-income-specific approaches to DG.

Field Test Results of an Innovative PV/T Collector for Solar Domestic Hot Water

Presently, only a limited number of hybrid collectors and systems are available. Hybrid solar PV/T has the potential to become a major player in the renewable energy sector, but one of the most important barriers is the inexistence of a proven track record in terms of reliability and performance. In order to address this issue, an in-depth study was realized to monitor in-field energy performances of two solar domestic hot water installations near Lyon (France). The two sites was equipped with an innovative unglazed PV-T collector, the DualSun technology.With 9m2 of DualSun collectors, hot water needs for 4 people were 91% covered from May to September, with an average annual coverage of 50%. These results are really similar to classical solar thermal performances. Temperatures in the used panels never exceeded 68 °C, demonstrating that there is no risk of overheating nor PV encapsulation degradations. The obtained performances were as high as expected from the Solar Keymark normative coefficients.

Comparative Analysis of Overheating Prevention and Stagnation Handling Measures for Photovoltaic-thermal (PV-T) Systems

The stagnation temperatures experienced by glazed photovoltaic-thermal (PV-T) collectors pose a threat to their performance and longevity, in part due to the limited temperature stability of ethylene vinyl acetate (EVA) encapsulants. In order to identify suitable solutions for application in residential solar domestic hot water (SDHW) PV-T systems, a comparative analysis of known overheating prevention and stagnation handling measures was conducted and dynamic simulations were used to support the analysis. While no measure was found to comply with all desirable goals including reliability, implementation and operation costs, integral venting mechanisms were identified as the most promising among the control systems reviewed. Moreover, active collector heat dumping and automatic collector shading led to minimal electrical efficiency increases and decreases, respectively, while purging tank water when the collector overheats was found to be ineffective due to delays in the start of circulation.

Performance analysis of solar cogeneration system with different integration strategies for potable water and domestic hot water production

A novel solar thermal cogeneration system featuring the provision of potable water with membrane distillation in combination with domestic hot water supply has been developed and experimentally analyzed. The system integrates evacuated tube collectors, thermal storage, membrane distillation unit, and heat exchangers with the overall goals of maximizing the two outputs while minimizing costs for the given design conditions. Experiments were conducted during one month’s operation at AURAK’s facility in UAE, with average peak global irradiation levels of 650W/m2. System performance was determined for three integration strategies, all utilizing brackish water (typical conductivity of 20,000μs/cm) as a feedstock: Thermal store integration (TSI), which resembles a conventional indirect solar domestic hot water system; Direct solar integration (DSI) connecting collectors directly to the membrane distillation unit without thermal storage; and Direct solar with thermal store integration (DSTSI), a combination of these two approaches. The DSTSI strategy offered the best performance given its operational flexibility. Here the maximum distillate productivity was 43L/day for a total gross solar collector area of 96m2. In terms of simultaneous hot water production, 277kWh/day was achieved with this configuration. An economic analysis shows that the DSTSI strategy has a payback period of 3.9years with net cumulative savings of $325,000 during the 20year system lifetime.

Wind Assisted Circulation Vs Forced Circulation Domestic Solar Hot Water Systems with Co2Filled Collector

The main focus of this study is to escalate the performance of the solar collector panel with minimal modification in a domestic solar hot water system. The conventional solar collector panel is filled with CO2 gas and the experiments were conducted for both forced circulation system (FCS) and wind assisted circulation system (WACS). The results were compared to the same systems when they are operated without filling CO2 in the solar collector panel. The hourly average efficiency seemed to be responding with respect to incident solar radiation and it corresponds to increased circulation rate of water across the collector. It was evident that the daily average system efficiency was enhanced by 3.27% and 3.6% for FCS and WACS mode of operation with CO2 filled collector panel. Comparative study between FCS and WACS modes of operation with and without CO2 reveal that WACS with CO2 gas filled in the collector plane under normal atmospheric pressure (WACS-CO2) appears be a better alternative to replace the FCS and WACS since WACS-CO2 does not require electricity for its operation and can be installed in remote areas with limited or no supply of electricity.

Investigation and Modelling of the Centralized Solar Domestic Hot Water System in Residential Buildings

The solar domestic hot water system is currently widely applied as a building energy saving technique. Various forms of the system exist, which should be properly applied in practical engineering. It is significant to study the adaptability of the solar domestic hot water system as it accounts for the building energy consumption in a non-negligible proportion, which is still scarcely studied based on field measurements in current research. This study figured out the performance of the centralized solar domestic hot water system through field measurements, as well as built a sub-hourly simulation model, which has been validated with the field measurement. The model was used to analyse the influence of several factors on the energy consumption of the system, such as the water supply strategy, the use modes of occupants, the thermal insulation of pipes, and the forms of the system. The adaptability of the system was concluded.

Effect of thermal stratification on energy and exergy in vertical mantled heat exchanger

In the present study, effect of thermal stratification on energy and exergy storage amount in vertical mantled hot water storage tanks, which are used common in solar domestic hot water systems, has been investigated. Placing obstacle inside the tank has enhanced thermal stratification and so energy and exergy of the tank. A commercial vertical mantled hot water tank was selected for experimental study. Four different obstacles were placed in four different heights from the tank bottom. Effect of the obstacle types and positions on energy and exergy storage capacity of tank has been researched experimentally. As a result, placing obstacles inside tank has enhanced the energy and exergy of tank. The best increasing has been seen for Obstacle A in Y/H = 0.250 position. In this situation energy amount increased 1.86 times and exergy amount increased 2.02 times higher than ordinary tank.

Feasibility analysis of solar combi-system for simultaneous production of pure drinking water via membrane distillation and domestic hot water for single-family villa: pilot plant setup in Dubai

This paper presents the feasibility study of installation of a solar-driven integrated MD desalination system for simultaneous production of pure drinking water and solar domestic hot water in United Arab Emirates (UAE) for a single-family villa comprising of 4–5 persons. In order to satisfy the current and future demand of water for domestic purposes, the desalination of seawater is considered to be one of the most effective and strategic technique in UAE. The stress on the underground water aquifers, rapid industrial growth, and increase in urban population in UAE results in the tremendous increase in fresh water demand during the past few decades. Since the local municipalities also provide the desalinated fresh water to the people but they mostly rely on bottled water for drinking purpose. In this paper, the pilot setup plant is designed, commissioned, and installed on site in UAE using air gap membrane distillation desalination process to fulfill the demand of 15–25 L/d of pure drinking water and 250 L/d of domestic hot water for a single-family villa. Experimental analyses have been performed on this setup during summer on flat plate solar collectors having different aperture areas (Experiments have been performed for aperture area of 11.9 m2 in this research study for feasibility purpose). The average hot-side temperature ranges from 50 to 70°C and average cold-side temperature of 35°C.

How significant is the stability of the radiative regime when the best operation of solar DHW systems is evaluated?

A typical Solar Domestic Hot Water (SDHW) system consisting of solar collectors and a water storage tank operating in fully mixed regime is considered. Optimal control operation is assumed. The objective is to maximize the net daily heat provided by the solar collectors. Meteorological data measured during year 2009 in Timisoara (Romania, Southeastern Europe) are used. Several days with more or less stable radiative regime are selected. Most results correspond to summer and spring days with daily relative sunshine larger than 0.4. The pumping energy increases by decreasing the daily relative sunshine. During more stable days the pump in the primary circuit operates a longer period of time than during less stable days. The opportunity to use the SDHW system for replacing a classical energy source depends on the stability of the radiative regime. The conclusion is that the dependence of SDHW systems' performance on the stability of the radiative regime is a complicate function of the specific performance indicator and the available amount of solar energy.

The effect of measurement time resolution on the peak time power demand reduction potential of domestic solar hot water systems

The electrical resistance showerhead is the water heating technology used in over 70% of the Brazilian dwellings. These high power (5–8 kW are typical) instant heating devices contribute significantly to the demand peak in the early evening, and are a major burden to distribution utilities in Brazil. The objective of this paper is to define consumer showering patterns, and analyze the influence of the power demand measurement time resolution on the potential of peak time power demand reduction provided by Domestic Solar Hot Water (DSHW) systems. Results show that the measurement of electrical power demand in a 15-min interval is not adequate to evaluate and verify the benefits provided by the use of DSHW systems, because it heavily underestimates the active power demand reduction at peak hours. This work suggests that a 1-min time resolution can be considered appropriate to assess the potential impacts caused by the use of DSHW systems on the peak hours demand reduction.