Domestic Hot Water Applications Research Articles

Digital Mapping of Techno-Economic Performance of a Water-Based Solar Photovoltaic/Thermal (PVT) System for Buildings over Large Geographical Cities

Solar photovoltaic thermal (PVT) is an emerging technology capable of producing electrical and thermal energy using a single collector. However, to achieve larger market penetration of this technology, it is imperative to have an understanding of the energetic performance for different climatic conditions and the economic performance under various financial scenarios. This paper thus presents a techno-economic evaluation of a typical water-based PVT system for a single-family house to generate electricity and domestic hot water applications in 85 locations worldwide. The simulations are performed using a validated tool with one-hour time step for output. The thermal performance of the collector is evaluated using energy utilization ratio and exergy efficiency as key performance indicators, which are further visualized by the digital mapping approach. The economic performance is assessed using net present value and payback period under two financial scenarios: (1) total system cost as a capital investment in the first year; (2) only 25% of total system cost is a capital investment and the remaining 75% investment is considered for a financing period with a certain interest rate. The results show that such a PVT system has better energy and exergy performance for the locations with a low annual ambient temperature and vice versa. Furthermore, it is seen that the system boundaries, such as load profile, hot water storage volume, etc., can have a significant effect on the annual energy production of the system. Economic analysis indicates that the average net present values per unit collector area are 1800 and 2200 EUR, respectively, among the 85 cities for financial model 1 and financial model 2. Nevertheless, from the payback period point of view, financial model 1 is recommended for locations with high interest rate. The study is helpful to set an understanding of general factors influencing the techno-economic performance dynamics of PVT systems for various locations.

Performance Evaluation of a Modified Compound Parabolic Concentrating Collector with Varying Concentration Ratio

Thermosyphon based evacuated tube solar collectors have seen exponential growth in the last few decades for domestic hot water applications. The output temperatures of such collectors can be increased by integrating evacuated tube receivers with compound parabolic concentrators (CPCs) to meet many industrial heat loads. This paper presents a new concept of increasing concentration of solar radiation along the length of an evacuated tube receiver coupled with a modified CPC with 52.5° and 38.5° acceptance half-angles at the inlet and outlet sides respectively. The effect of varying concentration ratio in the longitudinal direction is analyzed in terms of increasing heat flux and temperature gradient between the inlet and outlet of the receiver. Monte Carlo ray-tracing analysis confirmed a gradual increase of power density along the length of the receiver paired with the modified CPC. The daily average energy collection by the modified CPC was comparable with that of equal aperture area CPC with a fixed concentration ratio along its length. The experimental results demonstrated about a 15% higher temperature gradient in the case of CPC with a varying concentration ratio as compared to equal aperture area CPC with a fixed concentration ratio single-phase thermosyphon flow.

Long-term thermophysical behavior of paraffin wax and paraffin wax/polyaniline (PANI) composite phase change materials

Abstract Phase change Material (PCM) has immense potential in the field of energy storage due to its latent heat capacity. In this study, accelerated thermal cycling is performed on Paraffin wax (PW) and Paraffin Wax/Polyaniline (PWP-1) composite up to 3000 cycles to evaluate its durability. The latent heat of PW reduced by 6.7% at the end of 3000 cycles, whereas the composite PWP-1 reduced by 9.8% but PWP-1 had higher latent heat values compared to PW. The melting temperature showed an increment with the number of thermal cycles with 11.7% and 23% increment for PW and PWP-1 composite respectively. Thermogravimetric analysis showed very little change in the weight degradation after thermal cycling, but the Fourier Transform Infrared Spectrometry showed a new absorption peak for PW and PWP-1, which may be due to the formation of carbon/char. Visual and microscopic images of the samples are analyzed. From the study it can be concluded that PWP-1 composite has better thermal reliability in terms of latent heat as compared to PW and can be used in the field of solar application like flat plate solar collector, photovoltaic thermal system and low concentrated photovoltaic thermal systems for the production of domestic hot water application.

Parametric characterization of a full-scale plate-based latent heat thermal energy storage system

Thermal energy storage is a key technology to overcome the structural mismatch between energy supply and demand in thermal energy systems. Its use is essential for a proper integration of renewable or efficient technologies in day-to-day applications, such as heating and domestic hot water. Latent heat thermal energy storage, which is based on the use of phase change materials, has been studied in depth in the literature due to its advantages over conventional storage methods. It can provide a higher compactness and lower thermal losses than traditional sensible heat storage systems.However, to optimize the operation of latent thermal energy storage systems into heating and domestic hot water applications, it is still required to obtain additional information on the effects of the system parameters that can be easily altered and/or controlled. These parameters mainly are: operating temperature range, mass-flow rate and prototype length. This paper aims at evaluating these parameters for an innovative plate-based latent thermal energy storage system previously developed by the authors. The used phase change material is a commercial paraffin wax (RT60) with an approximated phase change temperature of 58 °C.A full-scale experimental setup was developed and forty-two experimental tests were conducted. The tests combined six temperature ranges from 45 to 70 °C, three mass-flow rates from 1.8 to 3 l/min and two module arrangements: a single prototype and two prototypes connected in series.The results showed that, for a specified temperature difference between the charging and discharging inlet temperature, the stored energy depended strongly on the selected temperature range. The thermal power during charging and discharging was strongly affected by the difference between the inlet temperature and the phase change temperature of the PCM. Higher mass-flow rates linearly reduced the required time for completion of charging or discharging. Finally, the addition of a second LHTES prototype in series enhanced the storage capacity of the system, but also increased the required time to complete the process.The obtained results can aid the proper design and operation of the system under specific constraints imposed by a specific application. They are also valuable as a starting point for the design of new plate-based LHTES systems, considering both different geometries and phase change materials.

Experimental Investigation of Thermal Performance Evaluation of Solar Flat Plate Collector

A flat plate collector is a simple and widely used device for space heating, domestic and industrial hot water applications. In this study, a device is fabricated and installed in Sultanpur for conducting experiments on flat plate collector to obtain the collector efficiency, rate of fluid heating and cooling and the temperature difference of fluid at inlet and outlet of the collector. The study made an attempt to enhance efficiency as compared to the normal flat plate collector by improving the collector design and varying the tilt angle, and mass flow rates. From the experiments, it was concluded that the efficiency is directly proportional to mass flow rate and inversely proportional to the wind speed. It was also found that factor like plate absorptive, glass cover emissivity, number of the glass cover and tilt angle and atmospheric condition affect the performance of flat plate collector.

Investigations of speed controlled heat pumps for NZEB in IEA HPT Annex 49

Speed controlled heat pumps are entering more and more the heat pump markets for space heating and domestic hot water (DHW) applications. In this study the design and control of speed controlled air-to-water heat pumps in net zero energy buildings are investigated by simulations of a single family house. The results confirm an increase in efficiency by using a speed controlled heat pump compared to an on/off controlled heat pump both in heating mode and in DHW mode. Further on, the correct sizing and control of a speed controlled heat pump is also important to reach a high efficiency.

An experimental investigation of the heat transfer and energy storage characteristics of a compact latent heat thermal energy storage system for domestic hot water applications

This paper presents the experimental performance analysis of a latent heat thermal energy storage system (LHTESS) designed for domestic hot water (DHW) applications. The designed, fabricated and characterised thermal store comprised of a vertically oriented multi-pass tube heat exchanger in a rectangular cross-section container filled with phase change material (PCM) paraffin wax RT44HC. The experimental investigation evaluated the heat transfer within the system, measured the transient temperature distribution, determined the cumulative thermal energy stored, charging and discharging time and the instantaneous charging and discharging power. The experimental work was conducted under controlled experimental conditions using different heat transfer fluid (HTF) inlet temperatures and different volume flow rates for store charging and discharging. It was found that during charging natural convection in the melt played a significant role. During discharging thermal conduction dominates and natural convection has an insignificant impact on the LHTESS performance. This is due to the development of a solid layer of PCM around the heat transfer tubes which increases the thermal resistance and reduces heat transfer to the liquid PCM. Higher HTF inlet temperature during charging significantly decreased store charging time. Increasing HTF inlet temperature from 60 to 70 °C shortened the charging time by 3.5 h, a further increase to 80 °C decreased melting time by a further 2 h.This study illustrates the extent to which LHTESS, and heat exchanger designs need to be improved to meet the desired charge/discharge time requirements for most short-term storage applications and that a broad range of domestic and commercial heat demands can be fulfilled by assembling several LHTESS units to operate in parallel.

Hybrid thermal energy storage with phase change materials for solar domestic hot water applications: Direct versus indirect heat exchange systems

A numerical model is developed and validated to simulate the performance of sensible energy storage (water tank) and hybrid energy storage (water tank including phase change material “PCM” modules) integrated into solar domestic hot water (DHW) system. Two configurations with direct heat exchange and indirect heat exchange using immersed heat exchangers are explored. A novel comparison is presented between both systems based on the solar fraction, which denotes the solar thermal energy contribution to the load. The effect of the storage volume on the solar fraction of the system is studied during a typical spring day while providing the hot water demands for a single-family residence, assuming a dispersed daily draw profile. The numerical results showed the crucial role of thermal stratification induced in the storage system with direct heat exchange. The storage system with direct heat exchange operates with 18–23% larger solar fraction than that with immersed coil heat exchangers. Adding PCM modules in the water tank with 50% volume fraction can yield around 40% potential reduction in the storage volume. The melting temperature of the PCM must be carefully chosen to maximize the energy storage in the latent form, thus limit the large temperature fluctuations of the system.

Thermal property study of fatty acid mixture as bio-phase change material for solar thermal energy storage usage in domestic hot water application

• Thermal property study of Bio-PCM OM55 using TGA/DTA, TCA, and DSC. • T-history method for bulk Bio-PCM quantification and cooling rate curve used to know the transition range. • Effects of cooling rate and thermal mass on supercooling. • Determination of isothermal enthalpy gives the actual enthalpy of the PCM material for proper storage design. • Comparison between conventional DSC, Bulk sample T-history results and the supplier’s data. For the correct design, simulation and specific application of the latent heat thermal energy storage (LHTES) system, detailed evaluation of phase change material (PCM) properties are essential. Present study aims to analyze the thermal and volume dependent behavior of available organic Bio-PCM OM55, using conventional thermal gravimetric analyzer (TGA), thermal constant analyzer (TCA), differential scanning calorimeter (DSC) and in-house T-history method (THM). Execution of the mentioned thermal analysis outcome with significant information of OM55. TGA shows that OM55 is thermally stable within the operating temperature 45–60 °C, because the maximum permissible degradation temperature 154.6 °C is much higher than operating temperature range. The OM55 has considerable thermal conductivity compared to the existing PCM, which is already used in domestic solar water heating (DSWH) applications. The evaluation of transition temperature, isothermal enthalpy, and specific heat by THM are well compared with the DSC analysis. Comparison of DSC and THM analysis showed that the behavior of OM55 is volume independent. The overall study concluded that OM55 is a potential Bio-PCM. However, for the optimum amount of energy storage and discharge in OM55, it is recommended to operate the LHTES unit over a temperature range between 46–59 °C for domestic hot water application.

Optimizing production efficiencies of hot water units using building energy simulations - Trade-off between Legionella pneumophila contamination risk and energy efficiency

The energy needed for domestic hot water represents an important share in the total energy use of well-insulated and airtight buildings. One of the main reasons for this high energy demand is that hot water is produced at temperatures above 60°C to mitigate the risk of contaminating the hot water system with Legionella pneumophila. However, this elevated temperature is not necessary for most domestic hot water applications, and has a negative effect on the efficiency of hot water production units. A simulation model has been developed which proposes an alternative to this constant 60°C by predicting the Legionella pneumophila concentration dynamically throughout the hot water system. Based on this knowledge, a hot water controller is added to the simulation model that sets a lower hot water comfort temperature in combination with heat shocks. In this paper, the simulation model is used to estimate the energy saving potential in a case study building, at the level of the heat production system by reaching higher production efficiencies. Three different production units, namely an electric boiler, heat pump and solar collector have been investigated. The controller is expected to become an alternative for the current, energy intensive, high temperature tap water heating systems.

Exergy Analysis of Serpentine Thermosyphon Solar Water Heater

The performance of a solar hot water system is assessed for heat pump and domestic heating applications. Thermodynamic analysis on a serpentine-type thermosyphon flat-plate solar heater is conducted using the Second Law of thermodynamics. Exergetic optimization is first performed to determine the parameters for the maximum exergy efficiency using MATLAB optimization toolbox. Geometric parameters (collector surface area, dimensions, and pipe diameter), optical parameters (transmittance absorptance product), ambient temperature, solar irradiation and operating parameters (mass flow rate, fluid temperature, and overall heat transfer (loss) coefficient) are accounted for in the optimization scheme. The exergy efficiency at optimum condition is found to be 3.72%. The results are validated using experimental data and found to be in good agreement. The analysis is further extended to the influence of various operating parameters on the exergetic efficiency. It is observed that optical and thermal exergy losses contribute almost 20%, whereas approximately 77% exergy destruction is contributed by the thermal energy conversion. Exergy destruction due to pressure drop is found negligible. The result of this analysis can be used for designing and optimization of domestic heat pump system and hot water application.

Compact latent heat storage decarbonisation potential for domestic hot water and space heating applications in the UK

A performance comparison is presented for a domestic space and hot water heating system with a conventional gas boiler and an air source heat pump (ASHP) with latent heat storage, both with solar thermal collectors for a typical UK climate, to demonstrate the potential of phase change material based energy storage in active heating applications. The latent heat thermal storage system consisted of 10 modules with RT54HC comprising a total storage capacity of 14.75 kWh that provided 53% extra thermal storage capacity over the temperature range of 40–65 °C compared to a water only store. The simulations predicted a potential yearly CO2 reduction of 56%, and a yearly energy reduction of 76% when operating the heat pumps using the economy 10 electricity tariff i.e. a low tariff between 00.00 and 05.00 and 13.00–16.00 with current grid emission values compared to the conventional gas boiler system; successfully offsetting the electrical load to meet the required heat demand. Due to the high capital costs of the heat pump system with latent heat storage, its levelized cost of energy was 117.84£/MWh, compared to 69.66£/MWh for the gas boiler, on a 20-year life cycle.

Investigation of a gas-fuelled water heater based on combined power and heat pump cycles

In this paper, we propose a novel gas-fuelled hot water system based on combined power and heat pump cycles. The proposed system essentially integrates a premixed gas burner, an organic Rankine cycle (ORC) power plant, and an air source heat pump for supplying hot water. An ORC power plant generates mechanical power from the thermal energy produced from the combustion of natural gas in the burner. Subsequently, the generated power directly drives a vapour compression cycle heat pump. Cold tap water is heated by three heat exchangers in series to gradually increase its temperature. It is preheated in the condenser of the heat pump, then heated in the condenser of the ORC power plant, and finally further heated by the flue gas exiting from the burner in a post heater. The flue gas exiting the post heater will be mixed with ambient air to further extract its residual heat in the evaporator of the heat pump. A comprehensive numerical analysis has been presented in this paper, and the results show that the proposed system can achieve an overall fuel-to-heat efficiency up to 147% when the cold water is heated from 10 to 65 °C and the ambient air temperature is in the range of −5 to 5 °C. The research results demonstrated that the proposed technology has a great potential for domestic hot water applications.

Potential of cascaded phase change materials in enhancing the performance of solar domestic hot water systems

The current paper explores a multi-tank thermal storage system for multi-residential solar domestic hot water applications. The thermal storage system includes phase change materials (PCMs) of different melting temperatures incorporated in the tanks. The PCMs are introduced as vertical cylindrical modules and water flowing along the length of tank is used as the heat transfer fluid. An enthalpy porosity model was developed to solve for the phase change process within the PCM modules. The model was validated and verified with previous work and predictions were in good agreement (less than 5% deviation). The hybrid tank model was linked with the collector performance. Typical Canadian weather data and a dispersed demand profile for a multi-residential building were considered. The performance of the hybrid system was judged based on the maximum possible storage volume reduction compared to the water only system with the same benefit to the end user. PCM maintains cooler water temperature entering the collector which results in a reduction of collector losses and extension of pump activation time. This increases the delivered energy to the load and hence increases the solar fraction. It was found that cascading four 75 L tanks containing PCMs of melting temperatures 54 °C, 42 °C, 32 °C and 16 °C gives a similar solar fraction to that for a 630 L water only tank. The multi-tank hybrid system thus allowed for over 50% reduction in the required storage volume.

Performance analysis and multi-objective optimization of a hybrid photovoltaic/thermal collector for domestic hot water application

PV/T collector turns out to be a promising alternative for the traditional solar thermal collector for domestic hot water (DHW) application. In this paper, four comprehensive thermal and electrical models including unglazed PV/T, glazed PV/T, PV and flat plate thermal collector are established for the purpose of accurate long-term simulation and optimization. The detailed 2-D temperature distributions of the glazed and unglazed PV/T collector are illustrated and compared for the first time. Besides, the dynamic response characteristics of different collectors are discussed and compared. By running the models under a broad combination of the operating conditions, the comprehensive performances are obtained and found to be significantly influenced by flow rate, temperatures, radiation and wind speed. A multi-objective optimization model coupling TRNSYS and NSGA-II tool is established to study and optimize the PV/T DHW system for a complete year. The Pareto frontier of conflicting objectives (life cycle savings and prime energy saving efficiency) is obtained for the optimal system design. The mass flow rate on the Pareto frontier is between 0.0085 kg/s and 0.011 kg/s in this study. The optimal value of tank volume on Pareto frontier shows an equally scattering distribution between 99.5 L and 218.6Lfor a 2 m2 glazed PV/T collector.

Energy and exergy investigation of a carbon dioxide direct-expansion geothermal heat pump

In this paper, a numerical simulation model has been extended for analyzing the energy and exergy of direct-expansion geothermal heat pumps (DX-GHP) in domestic hot water and heating applications. Carbon dioxide (CO2) in transcritical cycle has been used as the refrigerant and the effects of the refrigerant pressure drop in the heat exchangers have been considered. The present numerical investigation gives the energy and exergy analysis under different operational conditions in two separate cases. In the first case, the ground heat exchanger loop length is considered to be constant and in the second case, the heating load is considered to be fix. With the aim of evaluation the system work performance, a detailed parametric investigation has been carried out on the effect of different parameters including the difference between soil temperature and the outlet temperature of the evaporator, the compressor speed, the temperature and the mass flow rate of inlet water to the gas cooler, the gas cooler length, the number of ground heat exchanger loops, and soil temperature. The results of this study include heating capacity, coefficient of performance (COP), exergy efficiency, ground heat exchanger loop length can be used for design and optimization of a DX-GHP.

High density polyethylene spheres with PCM for domestic hot water applications: Water tank and laboratory scale study

Renewable energy is a potential alternative energy provider with fewer CO2 emissions. However, the mismatch between energy supply and demand is the main disadvantage. Therefore, thermal energy storage becomes an essential technology for enhancing renewable energy efficiency and providing energy supply to the end user. In solar thermal energy systems, hot water tanks are widely used as sensible heat storage technology. Moreover, water storage usually requires large volumes and their improvement has been studied in terms of shape and arrangement. Latent heat storage materials are a potential technology for implementation in water storage tanks in order to reduce their volume and to enhance their efficiency. In this paper, the incorporation of shape high density polyethylene spheres with PCM into domestic hot water tanks is studied. Undesired results obtained in the water tank set-up lead the authors to analyse the PCM leakage in the laboratory. Laboratory analysis pointed out that the PCM-spheres must be thermally cycled and cleaned before their implementation in real application of domestic hot water in order to stabilize the PCM content inside the PCM- spheres.

Experimental testing of a hybrid sensible-latent heat storage system for domestic hot water applications

Aim of this work is to present the results of the testing of a small scale hybrid sensible/latent storage system (nominal volume 48.6dm3), consisting of water in which macro-encapsulated phase change materials (PCMs) are added. Two different PCMs were macro-encapsulated, a commercial paraffin and a hydrate salts mixture prepared in the CNR ITAE lab, and loaded inside the tank in order to be tested. Different volume ratios between the PCM and the water were tested. The tests were conducted simulating different domestic hot water draw-off profiles.The resulting data showed an appreciable increase of heat storage capacity per unit of volume, even for limited fractions of PCM employed, reaching up to 10% of heat storage increasing by 1.3dm3 of hydrate salts mixture added. Finally, the experimental results were used to test a numerical method of a PCM enhanced tank for dynamic plant simulations in ESP-r environment.

Determination of the phase diagram and main thermophysical properties of the erythritol–urea eutectic mixture for its use as a phase change material

The present article studies the binary mixture comprised by erythritol and urea for its application as a phase change material (PCM) in latent hat thermal energy storage systems for heating and domestic hot water applications. A sequential methodology was employed to evaluate the suitability of the mixture as a PCM. First, a preliminary evaluation was performed by means of the modeling of the phase diagram and by hot–stage microscopy. Afterwards, the results were refined with more precise techniques, such as Differential scanning calorimetry, the synthetic visual method and X-ray diffraction. The results showed a eutectic composition formed by 54.9% (w/w) erythritol and 45.1% (w/w) urea, with a melting temperature of 81.13°C and a latent melting enthalpy of 248J/g. The specific weight of the eutectic composition was measured at different temperatures. The density of the liquid PCM at 95°C presented a value of 1.282g/cm3 and thus, the proposed eutectic mixture shows a volumetric storage capacity of 318MJ/m3. These properties are suitable for the applications aimed. The current market price of the mixture is nearby 2.5€/kg, which can be regarded as reasonable. Since urea undergoes thermal degradation upon heating, a preliminary accelerated thermal cycling test was performed. After 85 thermal cycles, the enthalpy of the eutectic mixture was reduced by 3.6%. In contrast, pure urea showed a decrease of 75% of its initial storage ability. According to the results, the eutectic mixture formed by erythritol and urea can be a good candidate for use as a PCM in medium temperature range applications. Further research is required to evaluate the stability of the mixture on longer periods of use. The methodology employed has proven to be adequate for the development of new eutectic PCMs.

The calibration and validation of a model for predicting the performance of gas-fired tankless water heaters in domestic hot water applications

Gas-fired tankless water heaters are gaining popularity as a means of producing domestic hot water for residential applications. As they become more widespread, these devices are increasingly becoming important to researchers who are seeking models of these devices that can be used to make reasonable predictions of their energy consumption. This is especially true for the case of condensing tankless water heaters as they have been shown to be the most efficient method of producing domestic hot water. Current models of these devices do exist, however, the uncertainties of their predictions are unclear and require data collected from onerous field-trials for calibration.This article makes a contribution by introducing a new model of a condensing tankless water heater whose uncertainties are well described that can be calibrated by a less-onerous experimental program in a laboratory. The model was calibrated with data spanning a range of conditions: inlet temperatures between 10 and 23°C, water flow rates between 0.08 and 0.27Ls−1 and outlet temperatures between 36 and 48°C.The model predictions were validated against data emanating from a previous field-trial. Above domestic hot water consumption values of 15MJday−1, the coefficient of determination (r2 value) was 0.98. The average error (difference between the average model predictions and the measured values) was 3.0% (in relative terms) while the root mean square error was 4.2%. The maximum error for a single point was 8.7% at a domestic hot water consumption of approximately 30.2MJday−1.