Effect of inlet design on the performance of storage-type domestic electrical water heaters

The effect of inlet design on the performance of storage-type domestic electrical water heaters (EWHs) was experimentally investigated for energy conservation. Three different side-inlet geometries; namely wedged, perforated, and slotted pipe-inlets; were tested using two 50 L capacity EWHs of aspect ratios 1 and 2 and two discharge rates 5 and 10 L/min. The three inlet designs are successful in promoting good thermal stratification inside the storage tanks of the EWHs. However, the degree of mixing produced by each design is found to have a significant impact on the performance. Thermal performance is quantified in terms of discharge efficiency, extraction efficiency, and fraction of heat recoverable. The results show an excellent performance for the slotted inlet. Although performance measures of perforated inlet are slightly less, the slotted inlet is attractive as it is simpler to manufacture and the extra cost associated with adopting it inside the existing EWH models is justified by the substantial savings in electricity and water consumptions, which ultimately concern the users.     

Effects of design factors on the temperature profile of a partially charged thermal storage tank

Thermal storage is a key subsystem in any solar thermal application. It stores thermal energy during periods of high solar irradiation so it can be used when solar irradiation is low and during the night. The energy extraction efficiency is influenced by design and by such operational factors as the state of charge of the thermal storage under normal conditions, a solar thermal storage system is in a partially charged state. This study was aimed at developing charged thermal storage with emphasis on the effect of inlet design, storage tank wall material, and the presence of fluid dead zones.     

Integrated collector storage solar water heater: Temperature stratification

An analysis of the temperature stratification inside an Integrated Collector Storage Solar Water Heater (ICS-SWH) was carried out. The system takes the form of a rectangular-shaped box incorporating the solar collector and storage tank into a single unit and was optimised for simulation in Scottish weather conditions. A 3-month experimental study on the ICS-SWH was undertaken in order to provide empirical data for comparison with the computed results. Using a previously developed macro model; a number of improvements were made. The initial macro model was able to generate corresponding water bulk temperature in the collector with a given hourly incident solar radiation, ambient temperature and inlet water temperature and therefore able to predict ICS-SWH performance. The new model was able to compute the bulk water temperature variation in different SWH collectors for a given aspect ratio and the water temperature along the height of the collector (temperature stratification). Computed longitudinal temperature stratification results obtained were found to be in close agreement with the experimental data.     

Analytical and experimental investigation of thermal stratification in storage tanks

An analytical and experimental investigation of transient turbulent two-dimensional charging and discharging of a sensible heat storage tank has been conducted. Parametric studies showed that the turbulent mixing factor due to hydrodynamic disturbances at the inlet ports is the most significant item in the performance of thermal stratification storage tanks. Furthermore, the effect of the aspect ratio and convection at the walls in promoting stratification have been studied. Comparison with experimental data showed the capability of the present analytic approach to accommodate, with a satisfactory degree of accuracy, such problems.     

A comparison of experimental thermal stratification parameters for an oil/pebble-bed thermal energy storage (TES) system during charging

Six different experimental thermal stratification evaluation parameters during charging for an oil/pebble-bed TES system are presented. The six parameters are the temperature distribution along the height of the storage tank at different time intervals, the charging energy efficiency, the charging exergy efficiency, the stratification number, the Reynolds number and the Richardson number. These parameters are evaluated under six different experimental charging conditions. Temperature distribution along the height of the storage tank at different time intervals and the stratification number are parameters found to describe thermal stratification quantitatively adequately. On the other-hand, the charging exergy efficiency and the Reynolds number give important information about describing thermal stratification qualitatively and should be used with care. The charging energy efficiency and the Richardson number have no clear relationship with thermal stratification.     

An experimental and analytical study of thermal stratification in an enclosed water region due to thermal energy discharge

An experimental and analytical study of the time-dependent thermal stratification that arises in a water body due to the discharge of thermal energy into it has been carried out in detail. The nature of the thermal stratification and its dependence on the physical variables of the problem, particularly on the flow configuration, are studied. An analytical model, based on the experimental results, is developed, and the predicted profiles are found to be close to those obtained experimentally. The nature of the buoyancy-induced mixing that arises is studied in terms of the analytical model, and the underlying mechanisms of the heat transfer process are discussed.     

The development of a refrigeration compressor for domestic heat pump application

An experimental investigation has been conducted of a compressor originally designed for air-conditioning applications to determine how much its performance could be improved for use in domestic heat pumps. The compressor was a hermetic unit of nominal capacity 15.6 cm³, driven by a single phase electric motor. The application selected was for heating only, air to air heat pumps. Improvements have been achieved in both coefficient of performance (13 per cent) and heat output (7 per cent), as well as in ancillary matters such as lubrication, noise and power factor.     

Review of seasonal heat storage in large basins: Water tanks and gravel–water pits

In order to respond to climatic change, many efforts have been made to reduce harmful gas emissions. According to energy policies, an important goal is the implementation of renewable energy sources, as well as electrical and oil combustion savings through energy conservation. This paper focuses on an extensive review of the technologies developed, so far, for central solar heating systems employing seasonal sensible water storage in artificial large scale basins. Among technologies developed since the late 1970s, the use of underground spaces as an energy storage medium – Underground Thermal Energy Storage (UTES) – has been investigated and closely observed in experimental plants in many countries, most of them, as part of government programmes. These projects attempt to optimise technical and economic aspects within an international knowledge exchange; as a result, UTES is becoming a reliable option to save energy through energy conservation. Other alternatives to UTES include large water tanks and gravel–water pits, also called man-made or artificial aquifers. This implies developing this technology by construction and leaving natural aquifers untouched. The present article reviews most studies and results obtained in this particular area to show the technical and economical feasibility for each system and specifics problems occurred during construction and operation. Advantages and disadvantages are pointed out to compare both alternatives. The projects discussed have been carried out mainly in European states with some references to other countries.     

Energy conservation and payback periods of natural circulation type solar water heaters

Energy savings in relation to different fuels, namely firewood, coal, kerosene, LPG and electricity have been calculated for a pressurized natural circulation type solar water heater with blackboard paint and a selective surface on the absorber. The payback periods have been computed by considering 10% compound annual interest, 5% annual maintenance cost and 5% inflation per year in maintenance cost and fuel prices. The cash flow has also been worked out for both solar water heaters. The cash flow is more for a solar water heater with a selective surface. The payback periods are 2.08–8.67 years for a solar water heater with a selective surface and 2.13–8.96 years for a solar water heater with blackboard paint. The estimated life of a heater is about 15 years. This shows that use of a solar water heater for heating water is very economical.     

Heating and cooling of a hospital using solar energy coupled with seasonal thermal energy storage in an aquifer

A system is being designed, using solar energy in combination with Aquifer Thermal Energy Storage (ATES), that will conserve a major part of the oil and electricity used for heating or cooling the Cukurova University, Balcali Hospital in Adana, Turkey. The general objective of the system is to provide heating and cooling to the hospital by storing solar heat underground in summer and cold in winter. As the main source of cold energy, ventilation air at the hospital and surface water from the nearby Seyhan Lake will be used.     

Performance of an inverted flat plate solar collector

In this note an analysis of an inverted flat plate solar collector has been presented. Effect of various parameters, namely thickness of the insulation, air conductance between flowing water and top of the bottom insulation, length of the collector and flow velocity of water on the performance of the collector have been studied. Numerical calculations have been made for typical cold day in Delhi, namely 26 January 1980.     

Thermal response of a series- and parallel-connected solar energy storage to multi-day charge sequences

The thermal response of a multi-tank thermal storage was studied under variable charge conditions. Tests were conducted on an experimental apparatus that simulated the thermal charging of the storage system by a solar collector over predetermined (prescribed) daylong periods. The storage was assembled from three standard 270 L hot-water storage tanks each charged through coupled, side-arm, natural convection heat exchangers which were connected in either a series- or parallel-flow configuration. Both energy storage rates and tank temperature profiles were experimentally measured during charge periods representative of two consecutive clear days or combinations of a clear and overcast day. During this time, no draw-offs were conducted. Of particular interest was the effect of rising and falling charge-loop temperatures and collector-loop flow rate on storage tank stratification levels. Results of this study show that the series-connected thermal storage reached high levels of temperature stratification in the storage tanks during periods of rising charge temperatures and also limited destratification during periods of falling charge temperature. This feature is a consequence of the series-connected configuration that allowed sequential stratification to occur in the component tanks and energy to be distributed according to temperature level. This effect was not observed in the parallel charge configuration. A further aspect of the study investigated the effect of increasing charge-loop flow rate on the temperature distribution within the series-connected storage and showed that, at high flow rates, the temperature distributions were found to be similar to those obtained during parallel charging. A disadvantage of both the high-flow series-connected and parallel-connected multi-tank storage is that falling charge-loop temperatures, which normally occur in the afternoon, tend to mix and destratify the storage tanks.     

A review of the economical and optimum thermal insulation thickness for building applications

Energy conservation is an increasingly important issue for the residential sector. Therefore, attention towards the thermal performance of building materials, particularly thermal insulation systems for buildings, has grown in recent years. In this study, a literature review on determining the optimum thickness of the thermal insulation material in a building envelope and its effect on energy consumption was carried out. The results, the optimization procedures and the economic analysis methods used in the studies were presented comparatively. Additionally, a practical application on optimizing the insulation thickness was performed, and the effective parameters on the optimum value were investigated.     

Matrix circuit model for an electric double layer capacitor

A mathematical model of an electric double layer capacitor is developed treating the capacitance as a matrix instead of a scalar. Model explicitly demonstrates that when two electrodes are immersed an electrolyte and a potential difference applied, a stable double layer that store energy is created. It is suggested that supercapacitors could be modeled on the basis of capacitance matrices whose elements parameterize the geometry of the porous electrode.     

Energy conservation and emission reduction policies for the electric power industry in China

Because of China's increasingly limited energy supplies and serious environmental pollution, much attention has been paid to conserving energy and reducing emissions to help the country's economy achieve sustainable development. As the electric power industry is the largest consumer of coal resources in China and also emits high levels of air pollutants each year, the Chinese government has enacted many technical and economic policies for energy conservation and emission reduction in the last few years. These policies are summarized in this paper, along with relevant laws and medium- and long-term plans, all of which address ideas such as adjusting the power generation mix, promoting demand-side management, introducing energy-efficient scheduling, and installing desulfurization units. The paper also assesses the results of these policies by analyzing several key indicators of energy consumption and emissions. The analysis shows that although some progress has been made in conserving energy and reducing emissions, substantial work is still required for China to catch up with developed countries. Some suggestions for future work are provided.     

Loss analysis of thermal reservoirs for electrical energy storage schemes

The paper presents an analysis of thermodynamic losses in thermal reservoirs due to irreversible heat transfer and frictional effects. The focus is upon applications to large-scale electricity storage for which it is the loss in availability (or exergy) that is most relevant. Accordingly, results are presented as loss coefficients which are defined as the fractional loss of the entering availability. Only losses stemming from irreversibility are considered – heat losses to the surroundings are not included in the analysis. A number of simplifying assumptions have been adopted, but the results nonetheless clearly demonstrate the dependence of the losses on operating temperatures, reservoir geometry and mode of operation, and point the way towards methods of optimisation. Estimates for a typical installation suggest that the losses are not insignificant, particularly for one-off charge and discharge (i.e., for long-term storage), but remain acceptable for cyclic operation, so as to make the use of thermal reservoirs attractive for electricity storage schemes.     

Performance prediction of a coupled metal hydride based thermal energy storage system

The present study discusses the thermodynamic compatibility criteria for the selection of metal hydride pairs for the application in coupled metal hydride based thermal energy storage systems. These are closed systems comprising of two metal hydride beds – a primary bed for energy storage and a secondary bed for hydrogen storage. The performance of a coupled system is analyzed considering Mg₂Ni material for energy storage and LaNi₅ material for hydrogen storage. A 3-D model is developed and simulated using COMSOL Multiphysics® at charging and discharging temperatures of 300 °C and 230 °C, respectively. The LaNi₅ bed used for hydrogen storage is operated close to ambient temperature of 25 °C. The results of the first three consecutive cycles are presented. The thermal storage system achieved a volumetric energy storage density of 156 kWh m⁻³ at energy storage efficiency of 89.4% during third cycle.     

A concept for storing utility-scale electrical energy in the form of latent heat

A concept is introduced here for storing utility-scale electrical energy in the form of latent heat. The storage process utilizes a boiling refrigerant at sub-ambient temperatures to freeze a latent heat storage material using electrically driven compressors. Recovery of the latent heat for electrical generation then uses vapor expansion and condensation which essentially reverses the storage process. Sensible heat storage is incorporated into the cycle to efficiently implement the concept. Both energy storage and generation are carried out under steady flow closed-loop conditions where the T-s diagram is similar to a Rankine cycle. From a thermodynamic perspective, work is supplied to the system while heat is transferred to the surroundings from the latent heat store. The reverse process generates work while using heat supplied by the surroundings. An analysis with expander/compressor isentropic efficiencies and small temperature differentials for the heat transfer processes can give projected round trip efficiencies in the 50–60% range using a common refrigerant. One of the attractive features of this approach is the ability to use different ambient temperatures for storage and generation. Exploiting diurnal temperature differences or sources of low grade heat (50–90 °C) significantly increases the apparent round trip storage efficiency.     

Methodology for characterising domestic electrical demand by usage categories

Electricity consumption in the United Kingdom is continually growing with demand from the domestic sector a potential/major contribution to this increase in consumption. Although demand is increasing, little information exists on the domestic components that contribute to an increase in domestic energy consumption. Thus, a greater understanding on what is contributing to the increase in domestic energy usage is a pre-requisite to understand how it can be reduced in the future or, if not reduced, contained at its current level.     

Computer analysis,design and simulation of horizontal ground heat exchangers

This paper presents a new computerized procedure for dealing with the design of horizontal ground heat exchangers (HGHE). The computer program is based on the transient model of coupled nonlinear partial differential equations governing heat and mass flow in soils. The model is two-dimensional and delineates the operation of ground heat storage with the HGHE and such phenomena as freezing/thawing and drying/rewetting of soil moisture. Comprehensive climatological data, such as ambient temperature, solar radiation, wind velocity, rainfall, snowfall, snow characterstics, and water vapour pressure is used to simulate conditions at the ground surface over any required length of time. The package can be applied to any geographical location by changing climatic and soil data input. The designer has the possibility of selecting any of 12 types of soils from sand to clay, 12 commercial heat pumps, nine different configurations of the HGHE, 16 plastic pipes for ground coils, and 13 ground coil fluids. The program, however, does not calculate the length of the HGHE but it evaluates the thermodynamic performance of a ground heat pump system and provides comprehensive data on thermal and hydraulic conditions in ground heat storage. The length of the ground heat exchanger is obtained from a line source theory model or from site dimensions and pipe spacing. Computed results for ground heat exchanger operation correlate fairly well with experimental data. Simulation of temperature and moisture content in the ground for natural conditions (no heat extraction/deposition) showed a fair agreement with field data. The entire computer program is user-friendly, interactive, menu-driven, and written in FORTRAN 77.