Effects of stratification on the performance of solar air heating systems

A theoretical model is presented which predicts the effects of stratification in the rock bed storage unit of a solar air heating system. The results are presented in terms of a stratification coefficient which is defined to be the ratio of the actual useful energy gain to the energy gain that would be achieved if the rock bed were maintained at a uniform temperature. This stratification coefficient is shown to be a system constant which depends on only three dimensionless system parameters.     

Simulation of solar air heating at constant temperature

Solar space heating with warm air in typical air collectors and rock bed storage systems involves constant air flow rates and varying the temperature of supply to rooms and to storage. This practice results in undesirable fluctuations in comfort levels in the living space, excessive storage size, useful but inaccessible heat in storage, and unnecessarily high energy consumption for air circulation and auxiliary heat. These drawbacks can be avoided by use of a practical controller and variable speed fan to provide heated air from the collector at constant temperature and a continually varying flow rate. Collector manufacturer's data, confirmed by seasonal tests on a solar air heating system in Solar House II at Colorado State University, have been used in simulations at constant hot air supply temperatures of 40°, 50°, and 60°C, and at one typical constant flow rate of 49 kg/h per m² through a 50 m² collector and rock bed storage unit, providing approximately half the seasonal heating requirements of a residential building. Auxiliary heat requirements and fan power use in the 40°C and 50°C constant temperature operations were significantly reduced from the levels prevailing under constant flow conditions. Collection efficiency and solar heat supply at constant flow were slightly higher than values at the 60°C constant temperature level.     

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.     

Investigation of rock bed solar collector cum storage system

This paper presents an experimental investigation of a prototype rock bed solar collector. This collector consists of rocks in a galvanized iron box; the rocks are painted dull black and suitably glazed. The heat can be extracted by forced air convection. This system can act as a storage system, as well, when suitably constructed. The heat decay characteristic of the bed is also studied. A preliminary economic analysis of this air heater is presented.     

Performance study of a partitioned thermally stratified storage tank in a solar powered absorption air conditioning system

Accurate modeling of solar heating or cooling with storage generally requires an accounting of the stratification within such storage tank, since overall system performance is significantly affected by the storage temperature distribution. In this study, a simple one-dimensional multi-node approach, taking into account of the axial heat conduction between nodes, has been used to theoretically analyze temperature stratification in the thermal storage tank. The results indicate that, for less collector area, the heat removal factor plays a major role in increasing the system performance, than the thermal stratification. Also, an optimum ratio of tank volume over collector area exists for a solar powered absorption air conditioning system. This paper also reviews the state of the art on different kinds of variable inlet design, and a simple new inlet design (partitioning the tank) has been introduced to effect better thermal stratification in storage tank.     

Thermal performance and pressure drop of rock beds with large storage materials

In air-based solar heating systems, the fan power needed to overcome friction loss in rock beds can reduce the benefit of the system. The system performance of rock beds with large-sized storage materials that have comparably low friction loss is studied. A theoretical model of the heat transfer process within the rock bed is developed for large storage materials. In this model, the temperature within the materials is assumed to be distributed quadratically and symmetrically at their center. The relationship between the model parameter and the air flow rate was derived from experimental measurements for some large materials as well as the pressure drop through the bed. The energy performance of heat pump solar systems with rock beds of various storage materials are studied by the computer simulation under Japanese winter weather conditions. It is concluded that the possibility exists for some large-sized storage materials to have almost the same performance as small-sized materials for heat pump solar systems.     

固-固相变蓄热材料在太阳能高温空气集热系统中的应用研究

为利用太阳能获得稳定持续的高温空气工质,除了有效集热外,还需要解决因太阳辐射强度变化导致输出工质温度波动的问题。在性能优良的太阳能集热系统中采用蓄热技术是解决此问题的有效途径。根据给定的设计目标,研究将固-固相变蓄热材料季戊四醇应用到太阳能集热蓄热一体化的实验装置中。实验结果表明:按集热蓄热一体化思路设计的实验装置,集热单元能够输出最高温度超过220℃的高温空气,蓄热单元能够将高温空气的温度稳定在蓄热材料的相变温度附近。并且随着蓄热管级数的增加,空气出口温度稳定的时间就越长,为利用太阳能获得稳定持续的高温热媒工质奠定了基础。     

Numerical and experimental analysis of convection heat transfer in passive solar heating room with greenhouse and heat storage

In this paper, heat transfer and air flow in passive solar heating room with greenhouse and heat storage are studied. Thermal insulation of solar heating room has significant effects on temperature distribution and airflow in the heating chamber of this solar system. Heat transfer and air flow in a rock bed, which is used as solar absorber and storage layer, are also studied. If porosity is kept within certain range, increasing the rock size causes an increase of the capability of thermal storage and heating effects; increasing the porosity of thermal storage materials results in an increase of the bed temperature but a decrease of the rock mass. The specific heat capacity and thermal conductivity have a remarkable effect on the average temperature of rock bed. All these factors should be taken into account when designing a solar heating system.     

Economic design of a rock bed storage device for storing solar thermal energy

This study deals with the optimization of design and operational parameters of a rock bed thermal energy storage device coupled to a two pass single cover solar air heater, i.e., charging time (), rock bed size (flow length, H), and cross-sectional area for square cross section (A_R), rock size (D_R), air mass velocity per unit bed cross-sectional area (G), and void fraction (ε). The optimization has been accomplished by investigating the effects of the above parameters on the total energy stored and the cost per unit energy stored in the rock bed for the winter climatic conditions of Delhi.     

A feasibility study of solar heating in Iran

A single-glass, flat-plate solar collector for air heating is analyzed for an optimum tilt angle of 45° for Shiraz (29° 36′ N latitude, 52° 32′ E longitude, and elevation of 4500 ft). The absorbed and utilized solar energy, as well as the collector outlet air temperature, the glazing, and the blackened plate temperatures, are determined with respect to the incident solar energy, parametric with collector inlet air temperatures and flow rates and outside air temperature.A 10 ft² collector and an 8 ft³ rock storage are built to experimentally verify the analysis and obtain cost estimates. A 5000 ft² single-story building is considered for solar heating and economic evaluations. Based on an annual interest rate of 8 per cent amortization of the solar heating equipment over 15 yr, electrical energy costs of 3c/kWh, and fuel costs of $1·10 per 10⁶ B.t.u., the optimum collector area which results in minimum annual operating costs (of the solar heating system and the auxiliary heating unit) is determined. A net saving results because solar heating is employed. The feasibility study is extended to eleven other Iranian cities. It is found profitable to employ solar heating in cities with low annual rainfall and relatively cold winters. An effective evaporative cooling is obtained by spraying water over the rock storage during the summer.     

An analytic model for the long-term performance of solar air heating systems

An analytic model is presented for the prediction of the monthly and yearly thermal performance of solar air heating systems. The effects of stratification in packed rock bed storage are explicitly taken into account. An expression for monthly solar fraction that depends on the important system and climatic variables is derived, which affords accurate predictions compared to corresponding f-chart calculations. The value of our analytic model, vis-a-vis calculational tools such as f-chart, is discussed. The method is applicable to all solar collector types as well as to load distributions of other than 24 hr/day.     

Thermal performance of a combined packed bed–solar pond system—a numerical study

Solar ponds have recently become an important source of energy that is used in many different applications. The technology of the solar pond is based on storing solar energy in salt-gradient stratified zones. Many experimental and numerical investigations concerning the optimum operational conditions and economical feasibility of solar ponds have been published in the last few decades. In the present study a modified solar pond with a rock bed inserted at the bottom is suggested and investigated. In order to conduct this study and predict the thermal performance of the combined system, a one-dimensional transient numerical model is developed. The boundary conditions are based on measured ambient and ground temperatures at Kuwait city. The model is validated for standard plain salt-gradient solar ponds and is then used to examine the thermal performance of the combined storage system for different rock material and bed geometry. It has been shown that the storage temperature is remarkably increased when low thermal diffusivity rocks (such as Bakelite) are used in the packed bed. Meanwhile, when high conductive rocks are used, the thermal storage temperature considerably deteriorates and the temperature variations amplitudes are almost flattened out. The bed geometry also plays a significant role in the storage process. As expected, an appreciable gain in the storage temperature was obtained for thicker rock beds. Low porosity rock beds, as well, produce higher storage temperatures in the storage zone.     

Simulated energy and exergy analyses of the charging of an oil-pebble bed thermal energy storage system for a solar cooker

Energy balance equations are used to model the solar energy capture (SEC) system and the thermal energy storage (TES) system of a proposed indirect solar cooker. An oil-pebble bed is used as the TES material. Energy and exergy analyses are carried out using two different charging methods to predict the performance of the TES system. The first method charges the TES system at a constant flowrate. In the second method, the flowrate is made variable to maintain a constant charging temperature. A Simulink block model is developed to solve the energy balance equations and to perform energy and exergy analyses. Simulation results using the two methods indicate a greater degree of thermal stratification and energy stored when using constant-temperature charging than when using constant-flowrate charging. There are greater initial energy and exergy rates for the constant-flowrate method when the solar radiation is low. Energy efficiencies using both methods are comparable whilst the constant-temperature method results in greater exergy efficiency at higher levels of the solar radiation. Parametric results showing the effect of each charging method on the energy and exergy efficiencies are also presented.     

Experimental study of an inexpensive solar collector cum storage system for agricultural uses

This paper presents an experimental investigation of an Augmented Integral Rock System. Experiments have been performed on such a solar air heater fabricated at the Institute. Experimental observations of fluid temperature, energy storage and other measures of system performance with variation of air mass flow rate, number of glazings and depth of rock bed are presented. The heat decay characteristics of the rock bed, while using a night insulation cover in addition to glass covers, are also reported in detail.     

Experimental characterisation of a thermal energy storage system using temperature and power controlled charging

The experimental set-up and technical aspects for charging a thermal energy storage (TES) of a proposed solar cooker at constant temperature and variable electrical power are presented. The TES is developed using a packed pebble bed. An electrical hot plate simulates the concentrator which heats up oil circulating through a copper coil absorber charging the TES system. A computer program to acquire data for monitoring the storage system and to maintain a nearly constant outlet charging temperature is developed using Visual Basic. The input power to the hot plate is also controlled to simulate the variation of the daily solar radiation by using another Visual Basic program. A combined internal model control (IMC) and proportional, integral and derivative (PID) temperature control structure is tested on the TES system under varying conditions and its performance is reasonable within a few degrees of the set temperature points. Results of the charging experiments are used to characterise the storage system. The different experiments indicate various degrees of stratification in the storage tank.     

Experimental investigation on a combined sensible and latent heat storage system integrated with constant/varying (solar) heat sources

The objective of the present work is to investigate experimentally the thermal behavior of a packed bed of combined sensible and latent heat thermal energy storage (TES) unit. A TES unit is designed, constructed and integrated with constant temperature bath/solar collector to study the performance of the storage unit. The TES unit contains paraffin as phase change material (PCM) filled in spherical capsules, which are packed in an insulated cylindrical storage tank. The water used as heat transfer fluid (HTF) to transfer heat from the constant temperature bath/solar collector to the TES tank also acts as sensible heat storage (SHS) material. Charging experiments are carried out at constant and varying (solar energy) inlet fluid temperatures to examine the effects of inlet fluid temperature and flow rate of HTF on the performance of the storage unit. Discharging experiments are carried out by both continuous and batchwise processes to recover the stored heat. The significance of time wise variation of HTF and PCM temperatures during charging and discharging processes is discussed in detail and the performance parameters such as instantaneous heat stored and cumulative heat stored are also studied. The performance of the present system is compared with that of the conventional SHS system. It is found from the discharging experiments that the combined storage system employing batchwise discharging of hot water from the TES tank is best suited for applications where the requirement is intermittent.     

Numerical analysis of latent heat thermal energy storage using encapsulated phase change material for solar thermal power plant

Thermal energy storage improves the load stability and efficiency of solar thermal power plants by reducing fluctuations and intermittency inherent to solar radiation. This paper presents a numerical study on the transient response of packed bed latent heat thermal energy storage system in removing fluctuations in the heat transfer fluid (HTF) temperature during the charging and discharging period. The packed bed consisting of spherical shaped encapsulated phase change materials (PCMs) is integrated in an organic Rankine cycle-based solar thermal power plant for electricity generation. A comprehensive numerical model is developed using flow equations for HTF and two-temperature non-equilibrium energy equation for heat transfer, coupled with enthalpy method to account for phase change in PCM. Systematic parametric studies are performed to understand the effect of mass flow rate, inlet charging system, storage system dimension and encapsulation of the shell diameter on the dynamic behaviour of the storage system. The overall effectiveness and transient temperature difference in HTF temperature in a cycle are computed for different geometrical and operational parameters to evaluate the system performance. It is found that the ability of the latent heat thermal energy storage system to store and release energy is significantly improved by increasing mass flow rate and inlet charging temperature. The transient variation in the HTF temperature can be effectively reduced by decreasing porosity.     

Transient analytical solution of solar energy devices

This note presents a transient analytical solution of fluid temperature variation in solar energy devices, viz. suspended plate solar air heater [Fitzgibbons and Kline (1978), Fig. 1], parallel flat plate collector (Fig. 2), solar stills [Tiwari and Malik (1982), Fig. 3] and pebble bed storage [Duffie and Beckman, (1980)] etc. as a function of time and space coordinate. Till now no such analytical solutions are available for such types of devices, except periodic and numerical analysis.     

Technoeconomic optimization of a hybrid solar air-heating system

This paper presents a simple techno-economic model for a hybrid solar air-heating system based on water as the storage medium. The configuration of the system consists of a conventional solar air-heater, water tank for thermal storage, a unit which adjusts the higher air temperature (during peak sunshine hours) to the required limit (by mixing fresh air) and an arrangement for providing auxiliary energy if and when required. A thermostatically controlled electric heater is assumed to be the source of auxiliary energy, in the present calculations. In order to evaluate the performance of the system using the developed model numerical calculations have been made corresponding to the climate of Delhi, India. The calculations have been extended to obtain the optimized values of collector area and storage mass which correspond to the minimum value of useful energy. Numerical results show that the cost of useful energy obtained for optimized values of collector area and storage mass is much less than the cost of electrical heating.     

Performance analysis of a latent heat storage system with phase change material for new designed solar collectors in greenhouse heating

The continuous increase in the level of greenhouse gas emissions and the rise in fuel prices are the main driving forces behind the efforts for more effectively utilize various sources of renewable energy. In many parts of the world, direct solar radiation is considered to be one of the most prospective sources of energy. In this study, the thermal performance of a phase change thermal storage unit is analyzed and discussed. The storage unit is a component of ten pieced solar air collectors heating system being developed for space heating of a greenhouse and charging of PCM. CaCl₂6H₂O was used as PCM in thermal energy storage with a melting temperature of 29 °C. Hot air delivered by ten pieced solar air collector is passed through the PCM to charge the storage unit. The stored heat is utilized to heat ambient air before being admitted to a greenhouse. This study is based on experimental results of the PCM employed to analyze the transient thermal behavior of the storage unit during the charge and discharge periods. The proposed size of collectors integrated PCM provided about 18–23% of total daily thermal energy requirements of the greenhouse for 3–4 h, in comparison with the conventional heating device.