Solar Domestic Hot Water Research Articles

PCM storage for solar DHW: From an unfulfilled promise to a real benefit

The present numerical study is concerned with the use of phase change materials (PCMs) in solar-based domestic hot water (DHW) systems. During the last decade, the majority of the studies related to that issue concluded that the recourse to PCMs-based storage units was quite promising in order to enhance the overall performances of solar-based DHW systems. One recently interesting published numerical study (Talmatsky and Kribus, 2008), suggested though that this beneficial impact is not guaranteed since the gains observed over the day period brought by the presence of PCMs to store the solar energy were compensated by the losses undergone by the storage tank during the night. The origin of this absence of any beneficial impact of the use of PCMs in a DHW system has to be clearly understood in order to reconcile studies which indicated apparently contradictory findings. In that framework, the goal of the present contribution is to analyze the conditions under which such an absence of advantage of the use of PCMs in a DHW system were obtained in order to propose some possibilities of improvement for demonstrating the interest in using PCMs in solar-based DHW systems. Thus, the mathematical model based on the one reported in Talmatsky and Kribus (2008) is considered. This model describes the heat storage tank with PCM, collector, pump, controller and auxiliary heater. Realistic environmental conditions and typical end-user requirements are imposed.

Effect of brine flow rate on the performance of a spiral-jacketed thermal storage tank used for SDHW systems: A computational fluid dynamics study

Abstract This study numerically investigates the effect of the brine flow rate on the thermal performance of a spiral-jacketed thermal storage tank (TST) installed in a solar domestic hot water (SDHW) system. The spiral-jacketed TST is a TST with a mantle heat exchanger, consisting of a vertical, cylindrical water tank for energy storage and a spiral brine flow path attached to the tank wall for heat transfer. A computational fluid dynamics (CFD) model was constructed based on the actual geometry of a spiral-jacketed TST. In addition, the boundary conditions were defined by considering solar radiation and ambient temperature data that were measured during experimental operation of the SDHW system. The numerical results demonstrated that an increase in the brine flow rate enhances the thermal efficiency of the TST due to higher heat transfer coefficients in the spiral path, and also leads to reduced thermal stratification in the TST. On the other hand, a lower brine flow rate increased the heat transfer rate at the inlet of the spiral path near the top of the TST, which resulted in enhanced thermal stratification. The optimal range for the rate of brine flow rate is discussed with respect to the thermal efficiency of the TST and the required pumping power for brine circulation in the spiral flow path.

High performance storage composite for the enhancement of solar domestic hot water systems: Part 1: Storage material investigation

This work aims to evaluate the performance of a solar domestic hot water (SDHW) system including a latent storage material. The originality of our approach consists to place a composite made of compressed expanded natural graphite (CENG) and phase change material (PCM) directly inside a flat plate solar collector in order to replace the traditional copper-based solar absorber. According to this target, the study is composed of two steps: the composites preparation and characterization; and the analysis of the system to achieve optimal integration of the material in the process. The present paper is focused on the selection of the most promising composite to implement in the solar collector. In order to reach this objective, several composites based on CENG and various storage materials (paraffin, stearic acid, sodium acetate trihydrate and pentaglycerin) have been elaborated and characterized. The synthesis of all these measurements allowed us to select three composites whose characteristics match their integration into a solar thermal collector.

A comparison of the UK Standard Assessment Procedure and detailed simulation of solar energy systems for dwellings

The drive to reduce worldwide carbon emissions that are directly associated with dwellings and to achieve a zero carbon home dictates that renewable energy technologies will have an increasingly large role in the built environment. The Standard Assessment Procedure (SAP), formulated by the Building Research Establishment (BRE), is the UK Government's approved methodology for assessing the energy ratings of dwellings. This article presents an evaluation of the advantage given to SAP ratings by the domestic installation of typical photovoltaic (PV) and solar domestic hot-water (SDHW) systems in the UK. Comparable PV and SDHW systems will also be simulated with more detailed modelling packages. Results suggest that calculation variances can exist between the SAP methodology and detailed simulation methods, especially for higher performance systems that deviate from the default efficiency parameters.

Case study for active solar space heating and domestic hot water preparation in a passive house

Models for five components of an active solar heating system are described. The integrated model was implemented to Pirmasens Passive House (Rhineland Palatinate, Germany). Two series schemes of interconnection for the domestic hot preparation system and the space heating system, respectively, are analyzed. Rather similar results were obtained in both cases. The active solar system starts operation when the incoming solar irradiance exceeds a threshold value depending on the ambient temperature and the amount of thermal energy stored in the water tank. The combined effect of passive and active solar systems is not able to cover the space heating demand for the entire days of January. Most part of the collected solar energy is used for domestic hot water preparation. The results show that the series interconnection is unfavorable for the space heating system.

A methodological concept for phase change material selection based on multiple criteria decision analysis with and without fuzzy environment

Selection of proper phase change material (PCM) plays an important role towards the development of a latent heat thermal energy storage system. Selection of the phase change material is a difficult and restrained task due to the immense number of different available materials having different characteristics. One has to select such PCM which will give the desired thermal performance at minimum cost. This study deals with two Multiple Attribute Decision-Making (MADM) methods to solve PCM selection problem. These two methods are technique for order preference by similarity to ideal solution (TOPSIS) method and fuzzy TOPSIS method that uses linguistic variable presentation and fuzzy operation. Both the methods use an analytic hierarchy process (AHP) method to determine weights of the criteria. TOPSIS and fuzzy TOPSIS methods are used to obtain final ranking. A problem to evaluate the best choice of PCM used in solar domestic hot water system is considered here to demonstrate the effectiveness and feasibility of the proposed model. Empirical results showed that the proposed methods are viable approaches in solving PCM selection problem. TOPSIS is suitable for the use of precise performance ratings while the fuzzy TOPSIS is a preferred technique when the performance ratings are vague and inaccurate.

Heat loss characteristics for a typical solar domestic hot water storage

It is common practice to predict the performance of solar domestic hot water (SDHW) systems by computer simulation. This process relies on the accurate specification of the system's physical and thermal characteristics, and is often based on a number of simplifying assumptions. An important aspect of system performance is storage heat loss characteristics; however, these are often represented by an average heat loss coefficient or U-value that does not account for the complex geometry of the thermal storage or the interaction of the various inlet and outlet ports that may act as thermal conduits. In addition, most solar storage models assume that the tank temperature profile is one-dimensional and that conduction within the tank wall is negligible. To investigate these effects, tests were conducted on a typical thermal storage used in SDHW applications and included a cool-down test and a heat diffusion test sequence. The values derived from these test sequences were then compared to computer predictions based on estimated thermal properties. In addition, the basic assumptions typically used in the computer modelling of solar storage heat losses (e.g., one-dimensional temperature profiles, minimal tank wall conduction, uniform wall heat loss) were investigated, particularly in the context of a thermally stratified thermal storage.

Detailed analysis of the energy yield of systems with covered sheet-and-tube PVT collectors

Solar cells have a typical efficiency in the range of 5–20%, implying that 80% or more of the incident solar energy can be harvested in the form of heat and applied for low-temperature heating. In a PVT collector one tries to collect this heat. In this work, the electrical and thermal yield of solar domestic hot water systems with one-cover sheet-and-tube PVT collectors were considered. Objectives of the work were to understand the mechanisms determining these yields, to investigate measures to improve these yields and to investigate the yield consequences if various solar cell technologies are being used. The work was carried out using numerical simulations. A detailed quantitative understanding of all loss mechanisms was obtained, especially of those being inherent to the use of PVT collectors instead of PV modules and conventional thermal collectors. The annual electrical efficiencies of the PVT systems investigated were up to 14% (relative) lower compared to pure PV systems and the annual thermal efficiencies up to 19% (relative) lower compared to pure thermal collector systems. The loss of electrical efficiency is mainly caused by the relatively high fluid temperature. The loss of thermal efficiency is caused both by the high emissivity of the absorber and the withdrawal of electrical energy. However, both the loss of electrical and thermal efficiency can be reduced further by the application of anti-reflective coatings. The thermal efficiency can be improved by the application of a low-emissivity coating on the absorber, however at the cost of a reduced electrical efficiency.

Comparison of the dynamic and input–output methods in a solar domestic hot water system

The purpose of this work is to present a comparison between two standardized test methods for the evaluation of the energy performance of Solar Domestic Hot Water (SDHW) systems. The approaches investigated concern the Dynamic method (DST) and the Input–Output method (CSTG). These methods have been officially established as ISO standards, thus they have found wide dissemination, and their results are supposed to be equivalent to one another. A description of the operation principle of each method and the corresponding test procedures is presented, focusing on the aspects related to the practical implementation of these methods on a laboratory level. A Solar Domestic Hot Water system is tested according to both methods. On the basis of the measurements, the characteristic parameters of the SDHW system are calculated for each method and the long-term performance under typical meteorological conditions is estimated. Consequently, a comparative evaluation and analysis of the observed deviations between the results of the two methods are performed. Besides the quantitative comparison, this work has attempted to evaluate the two methods on a qualitative basis as well, by focusing on their functionality, their requirements regarding the number of test days considered and the physical interpretation of the results.

Conventional versus storage domestic solar hot water systems: A comparative performance study

This paper presents an experimental comparative performance study between a storage solar domestic hot water system and a conventional one. The storage system differs from the conventional one by integrating the collector and storage tank into one piece of equipment. The investigated storage collector consists of six 80-mm-outside diameter copper tubes connected in series to act as an absorber and a storage tank. Tests under different flow patterns are conducted to simulate different patterns of hot water consumption. The overall heat loss coefficient, useful heat gain, instantaneous efficiency, efficiency factor, heat removal factor and the temperature distribution along the pipe are determined using modified equations derived for the storage collector from the basic derivation of Hottel–Bliss–Whillier. A comparison between the experimental and theoretical results for the storage collector showed good agreements and that between the two systems showed the superiority of the storage system.

A truncated pyramid non-tracking type multipurpose domestic solar cooker/hot water system

For impressive dissemination of the solar thermal gazettes, it is imperative to keep on changing the device design features so as to cater to the different demands of diverse section of the society. Domestic solar hot water systems are not suitable for cooking and the capacity of domestic solar box type cookers for water heating is very low. We report truncated pyramid geometry based multipurpose solar device which could be used for domestic cooking as well as water heating. The device is designed, fabricated and tested. Cooking tests approved by Bureau of Indian Standards were performed in different seasons and the device was found to meet the requirement stipulated on two figures of merit. The performance of the design was also evaluated as a hot water system and the maximum efficiency was found to be 54%. The day-time and average night-time heat-loss coefficients were found to be 5.7 W/°C m 2 and, 3.74 W/C m 2, respectively, which are comparable to those of flat-plate collector based solar hot water systems. A simple economic analysis illustrate that this kind of multi-purpose design could be financially viable and physically useful.

Thirty years of domestic solar hot water systems use in Greece – energy and environmental benefits – future perspectives

The effort to reduce the dependence on imported crude oil in Greece, after the oil crises in the '70s, has resulted, among others, in a total installed area of 3.57 million m 2 solar collectors in 2007, making Greece one of the pioneers in the use of domestic solar hot water system (DSHWS) worldwide. In the present work, the contribution of DSHWS to the reduction of conventional energy and greenhouse gases and other air pollutant emissions in Greece from its early years in mid '70s up to now is assessed. DSHWS market penetration, solar system technological changes and development and demographic changes in association with the climatic conditions in all regions of the country have been taken into account in order to calculate energy conservation and emissions reduction. The results show that the conserved energy ranges from 21.27 GW h el (0.1% of the domestic sector energy use) in 1978 to 1513 GW h el (2.4%) in 2007, resulting in an abatement of CO 2 emissions, which for the year 2000 was 1.67 Mt, exceeding by 76% the objectives of the Greek Program of “Climatic Change”, which indicated savings of 0.95 Mt CO 2 for 2000. Moreover DSHWS maximum technical potential is assessed to be about three times the current installed area, showing that they can play an important role in energy end environmental policy of the country.

Characterization of a Thermosyphon Heat Exchanger for Solar Domestic Hot Water Systems

This paper presents a simplified test method that was developed to allow preconfigured solar domestic hot water systems that use natural convection/thermosyphon heat exchangers to be characterized. The results of this test method produce performance coefficients for simple empirical expressions that describe the fluid flow and heat transfer in the heat-exchange loop. These empirically derived coefficients can be used as an input to a general simulation routine that allows overall system performance to be determined for various loads and climatic conditions. To illustrate the test procedure, results are presented for a typical heat exchanger under a range of operational conditions.

Utilization of phase change materials in solar domestic hot water systems

Thermal energy storage systems which keep warm and cold water separated by means of gravitational stratification have been found to be attractive in low and medium temperature thermal storage applications due to their simplicity and low cost. This effect is known as thermal stratification, and has been studied experimentally thoughtfully. This system stores sensible heat in water for short term applications. Adding PCM (phase change material) modules at the top of the water tank would give the system a higher storage density and compensate heat loss in the top layer because of the latent heat of PCM. Tests were performed under real operating conditions in a complete solar heating system that was constructed at the University of Lleida, Spain. In this work, new PCM-graphite compounds with optimized thermal properties were used, such as 80:20 weight percent ratio mixtures of paraffin and stearic acid (PS), paraffin and palmitic acid (PP), and stearic acid and myristic acid (SM). The solar domestic hot water (SDHW) tank used in the experiments had a 150 L water capacity. Three modules with a cylindrical geometry with an outer diameter of 0.176 m and a height of 0.315 m were used. In the cooling experiments, the average tank water temperature dropped below the PCM melting temperature range in about 6–12 h. During reheating experiments, the PCM could increase the temperature of 14–36 L of water at the upper part of the SDHW tank by 3–4 °C. This effect took place in 10–15 min. It can be concluded that PS gave the best results for thermal performance enhancement of the SDHW tank (74% efficiency).

Comparative study of various optimization criteria for SDHWS and a suggestion for a new global evaluation

This study compares various optimization criteria for a solar domestic hot water system (SDHWS). First of all, we present the various parameters used to evaluate a SDHWS. We consider the energetic, exergetic, environmental (CO 2 emissions) and financial (life cycle cost) analysis. Various optimization criteria of a standard solar hot water system are then proposed. The optimized solutions are compared with a standard hot water system. The most suitable criteria take into account both energetic (therefore environmental) and financial evaluations. The most powerful solutions tend to increase the collector area – increasing the solar fraction during the mid-season – and reduce the tank volume, thereby decreasing the thermal losses and financial cost. Some of the usual evaluation criteria for SDHWs cannot be used as optimization criteria because they do not consider the auxiliary heater, resulting in inaccurate indications of the system’s performance. Therefore, it seemed important to propose a new evaluation method which integrates the life cycle savings, primary energy savings and CO 2 emission savings with regard to a referenced solution based on a radar diagram of these three fractions. This mode of representation is particularly useful when various auxiliary heaters are compared.

Discharge of a thermal storage tank using an immersed heat exchanger with an annular baffle

A number of solar domestic hot water systems and many combined space and water heating systems have heat exchangers placed directly in the storage fluid to charge and/or discharge the tank. Operation of the heat exchanger produces a buoyancy-driven flow within the storage fluid. With a view toward controlling the flow field to increase heat transfer, a cylindrical baffle is inserted in a 350 l cylindrical storage tank. The baffle creates a 40 mm annular gap adjacent to the tank wall. A 10 m-long, 0.3 m 2 copper coil heat exchanger is placed in the gap. The effects of the baffle on the transient heat transfer, delivered water temperature, heat exchanger effectiveness, and temperature distribution within the storage fluid are presented during discharge of initially thermally stratified and fully mixed storage tanks. The baffle increases the storage side convective heat transfer to the heat exchanger by 20%. This increase is attributed to higher storage fluid velocities across the heat exchanger.

PCM storage for solar DHW: An unfulfilled promise?

Phase change materials (PCM) have been repeatedly proposed for use in solar domestic hot water (DHW) systems. PCM storage designs have been proposed, but no detailed evaluation has been made of the actual contribution of the PCM to the total heat storage under typical end-use conditions. In this work annual simulations were done to compare the performance of a storage tank with PCM to a standard tank without PCM. A model was constructed to describe the heat storage tank with and without PCM, the collector, pump, controller and auxiliary heater. Realistic environmental conditions and typical end-user requirements were imposed. Annual simulations were carried out for different sites, load profiles, different PCM volume fractions, and different kinds of PCM. The results of all simulation scenarios indicate that, contrary to expectations, the use of PCM in the storage tank does not yield a significant benefit in energy provided to the end-user. The main reason for this undesirable effect is found to be increased heat losses during nighttime due to reheating of the water by the PCM.

Exergetic modeling and assessment of solar assisted domestic hot water tank integrated ground-source heat pump systems for residences

The present study deals with the exergetic modeling and performance evaluation of solar assisted domestic hot water tank integrated ground-source heat pump (GSHP) systems for residences for the first time to the best of the author's knowledge. The model is applied to a system, which mainly consists of (i) a water-to-water heat pump unit (ii) a ground heat exchanger system having two U-boreholes with an individual depth of 90 m, (iii) a solar collector system composing of rooftop thermal solar collectors with a total surface area of 12 m 2, (iv) a domestic hot water tank with a electrical supplementary heater, and (v) a floor heating system with a surface of 154 m 2, and (vi) circulating pumps. Exergy relations for each component of the system and the whole system are derived for performance assessment purposes, while the experimental and assumed values are utilized in the analysis. Exergy efficiency values on a product/fuel basis are found to be 72.33% for the GSHP unit, 14.53% for the solar domestic hot water system and 44.06% for the whole system at dead (reference) state values for 19 °C and 101.325 kPa. Exergetic COP values are obtained to be 0.245 and 0.201 for the GSHP unit and the whole system, respectively. The greatest irreversibility (exergy destruction) on the GSHP unit basis occurs in the condenser, followed by the compressor, expansion valve and evaporator.

Experimental and numerical study of thermal stratification in a mantle tank of a solar domestic hot water system

The simulation and the optimisation of the mantle tank of solar domestic hot water systems needs dynamic simulation over long periods of time (e.g. 1 year). A model for such a mantle tank was developed by using the zonal approach. The dimensions of the zones are determined based on physical considerations. A mixing coefficient is identified to model the water flow in the mantle heat exchanger. Comparisons of the results of temperatures distribution of the model and of experiments show a difference <7% for three positions of the inlet water flow in the mantle heat exchanger.

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.