Solar Heat Storage Research Articles

Design and Application of a Seasonal Solar Soil Heat Storage System Applied in Greenhouse Heating

A seasonal solar soil heat storage (SSSHS) system applied in greenhouse heating has been designed and introduced. The system consists of solar collector subsystem, soil heat storage subsystem, greenhouse heating subsystem, hydronic subsystem and control subsystem. By applying soil heat storage, solar energy stored in the soil under the greenhouse can be transferred and utilized in winter to realize the utilization of cross-seasonal energy. TRNSYS is used to simulate the process and effect in the system of the solar energy collection and soil heat storage in Shanghai, and the simulation is calibrated to improve the precision of the TRNSYS model. When the indoor air temperature of the greenhouse is kept at 12°C throughout the year, the energy saving by using the SSSHS system in Shanghai can be 46.2kWh/(m2∙year).

Thermocline Storage Filled with Structured Ceramics. Numerical Consistency of the Developed Numerical Model and First Observations

AbstractThe attraction of solar energy is greatly enhanced by the possibility of it being used during times of reduced or non-existent solar flux, such as weather induced intermittences or the darkness of the night. Therefore optimizing thermal storage for use in solar energy plants is crucial for the success of this sustainable energy source. Here we present a study of a structured bed filler dedicated to Thermocline type thermal storage, believed to outweigh the financial and thermal benefits of other systems currently in use such as packed bed Thermocline tanks. Several criterions such as Thermocline thickness and Thermocline centering are defined with the purpose of facilitating the assessment of the efficiency of the tank to complement the standard concepts of power output. A numerical model is developed that reduces to two dimensions the modeling of such a tank. The structure within the tank is designed to be built using simple bricks harboring rectangular channels through which the solar heat transfer and storage fluid will flow. The model is scrutinized and tested for physical robustness, and the results are presented in this paper. The consistency of the model is achieved within particular ranges for each physical variable.

Energy and exergy analysis of natural gas pressure reduction points equipped with solar heat and controllable heaters

In order to prevent gas hydrate forming, Natural gas has to be warmed up at natural gas pressure reduction point (usually called City Gate Station (CGS)) before pressure reduction takes place. The gas is warmed up using the bath type heater which burns the natural gas as fuel. Employing solar heat in CGSs had been previously proposed by the authors; however, there were two shortcomings in the previous articles. Firstly, the employed heater was not equipped with an automatic control system (instantaneous control could not be achieved) and secondly, the exergy destruction rate of the proposed system had not been compared with the conventional configuration of a CGS. This article presents the feasibility of employing solar heat storage system at CGSs equipped with controllable heaters. The proposed system is thoroughly evaluated in terms of fuel providence and exergy destruction rate and the results are compared with the conventional and the previous proposed configurations.

Design and experimental study of an industrialized sunspace with solar heat storage

Abstract This investigation deals with the thermal performance of two passive solar components. An attached sunspace with horizontal heat storage and another one with vertical thermal storage were designed in order to optimize the use of solar gain, its storage and distribution in an industrialized component. These sunspaces have been tested under real conditions, comparing their thermal performance with two commonly used components in residential buildings in Spain: a window and a double window making up an attached sunspace. Different series of experimental measurements were made in two test-cells exposed to outdoor conditions in Pamplona (Northern Spain). As a result, nine scenarios during winter 2011 and six ones during summer 2012 have been carried out, comparing all the prototypes two by two with different use modes. Resu**lts show that a sunspace with heat storage takes advantage of the solar energy and improves the indoor thermal performance of the adjacent room during winter in a better way than a window or a simple sunspace, and that it has also a better performance in summer. The best results in winter and summer were obtained when an appropriate use of the component is performed, in concordance with outdoor conditions; some thermal control actions for the good performance of these components are suggested.

Study and Analysis on the High Temperature Performance of Calcined Bauxite

In order to further broaden the application field of calcined bauxite, using XRF and other testing technology to study and analysis on the high temperature performance of calcined bauxite in this paper. The results showed that the sintering temperature range was 1300 ~ 1650°C, the firing line shrinkage, bulk density, bending strength, heat capacity and thermal conductivity was increased gradually, the water absorption and porosity was decreased, the water absorption, volume density and flexural strength of the sample fired at 1650°C were 0.93%, 3.20 g·cm-3 and 182.23 MPa, the specific heat and thermal conductivity were 0.83 kJ·(kg·K)-1 and 9.21 W·(m·K)-1. Due to the thermal performance of calcined bauxite is optimal, and is expected to be used for the field of high temperature solar thermal heat storage material.

Effect of heat storage and fuel price on energy management and economics of micro CCHP cogeneration systems

In the present work, a typical combined cooling, heating and power (CCHP) system comprised of boiler, flat solar collectors, absorption chiller and heat storage tank was investigated. The described system was considered to supply the given electricity, cooling and heating demand of a residential building; with heating and cooling needs of 100 and 50 kW, respectively. To find the optimum hybrid configurations with high reliability, low costs, low fuel consumption and emissions, a computer program was provided by authors in FORTRAN language. Different fuel prices were considered in the present work. The results indicated that the optimal operation strategy changes with Boiler and NGG fuel prices while it also changes with increasing the number of solar collectors, heat storage capacity and consequently decreasing total annual emission.

A Novel Process for Solar Hydrogen Production Based on Water Electrolysis in Alkali Molten Carbonates

High temperature water electrolysis has a tremendous potential in enabling a cost-effective and sustainable production of synthetic fuels like hydrogen by using renewable heat and electricity from concentrating solar thermal (CSP) plants [1]. Most efforts in this research area are currently directed towards investigating solid oxide electrolysis cells (SOECs) based on fuel cell technology [2,3], although this approach is not optimal for integration with CSP plants because of electrolysis temperatures (i.e., above 800°C) significantly exceeding those of most solar heat storage systems. In order to improve thermal matching with current solar thermal fluids, development of electrolysis processes within the 500-600°C range would be a desirable option. In this context, molten salts could be seen as an ideal medium to lower process temperatures with respect to solid oxide electrolyzers since overall ionic conductivity and transport numbers of liquid salts are usually higher than solid-type electrolytes. Recently, alkali molten carbonate salts have gained a return of attention as a promising and attractive electrolyte for electrochemical conversion processes of mineral ores and CO2 gas at moderate temperatures [4,5]. At the same time, water electrolysis in alkali molten carbonates has been recently mentioned as a feasible process [2], although no systematic studies on this process are available in literature. The overall electrolysis reactions can be written as: cathode, H2O+CO2 + 2e = H2+CO3 2- (1)anode, CO3 2- = CO2 +0.5O2 +2e (2) The anode gas mixture made of 33 % O2 + 67% CO2 is ideal for oxy-combustion processes. Thus, a zero CO2 emission system could be realized by integrating a molten carbonate electrolysis with an oxycombustion process. The combined process could enable a CO2 closed-loop recycling scheme with CO2 capture. Part of the post-combustion CO2 could be, in fact, re-injected to the cathode, whereas the excess CO2 could be easily captured.From a mechanistic point of view, the exact cathode reduction reaction is not well understood. Although the cathode reaction can be interpreted in terms of a conventional water reduction process (eq. (1)), there are indications in literature that bicarbonate anions are likely the reactive species rather than dissolved water according to [6]: 2HCO3 - + 2e =H2+2CO3 2- (3) The presence of bicarbonate is possible because, in presence of a water-containing atmosphere, molten carbonates cannot be considered as a pure solvent, but rather as a mixture of three anionic constituents, namely carbonate, bicarbonate and hydroxide ions, according to the following chemical equilibrium: H2O+CO3 2- = HCO3 - + OH- (4) Thermodynamic calculations indicate that bicarbonate concentration is lower than hydroxide, but it is not negligible below 600°C. Initial lab-scale work has confirmed the effective presence of hydrogen as main cathode reaction product in a electrolysis conducted in ternary alkali carbonate eutectic mixture Li2CO3-Na2CO3-K2CO3 (43.5-31.5-25.0 mol %) despite the relatively low values of steady-state current density observed within the 525-600°C range (see Fig.1). Cyclic voltammetry experiments are also underway to study in more detail kinetics and mechanisms of the cathodic reaction, which is an essential aspect for interpreting and modeling the overall electrolysis process. Detailed results of these exploratory investigations will be shown at the time of Conference.

Syngas and Hydrogen Production by Cyclic Redox of ZrO2-Supported CeO2 in a Volumetric Receiver-Reactor

In order to utilize sustainable solar energy, cyclic operations of syngas production by methane reforming (reduction) and subsequent hydrogen production by water splitting (oxidation) were performed by using simulated solar-light irradiation to ZrO2-supported CeO2 particles which were coated on a SiC ceramic foam disk. This redox process is a promising chemical pathway for storage and transportation of solar heat by converting solar energy to chemical energy. By properly adjusting the methane reforming time, carbon deposition due to the undesirable methane decomposition could be avoided. The produced syngas had the H2/CO ratio of 2.0, which is suitable for the Fischer–Tropsch synthesis or methanol synthesis, and the produced pure hydrogen can be used for fuel cells. When the cyclic reactions were repeated several times at two temperatures (800 °C, 900 °C), the conversion of CeO2 and the H2 yield were reasonable and were maintained nearly constant from the second cycle, exhibiting good stability of the redox process.

Thermodynamic analysis and experimental study of solid/gas reactor operating in open mode

Thermochemical storage with moist air is a promising way to achieve seasonal storage of solar heat. This paper investigates the link between outlet and inlet airflow conditions in the case of hydration/dehydration of thermochemical processes operating with moist air at atmospheric pressure. A thermodynamic analysis of a solid/gas reactor operating in open mode is performed. That leads to a thermodynamic relation called charge–discharge operating line. Experimental tests have been carried out with two different reactive salts in order to prove the validity of the charge–discharge operating line. Experimental results provide a close match between the theoretical approach of the charge–discharge operating line and the measurements. Based on this relation, a reactor performance criterion, called reaction effectiveness, is defined. Both operating line and effectiveness stand as efficient design tools to predict performances of the reactor and to guide R&D choices to enhance them.

Exploitation of thermochemical cycles based on solid oxide redox systems for thermochemical storage of solar heat. Part 1: Testing of cobalt oxide-based powders

Thermochemical storage of solar heat exploits the enthalpy effects of reversible chemical reactions for the storage of solar energy. Among the possible reversible gas–solid chemical reactions, utilization of a pair of reduction–oxidation (redox) reactions of solid oxides of multivalent metals can be directly coupled to Concentrated Solar Power (CSP) plants employing air as the heat transfer fluid avoiding thus the need for separate heat exchangers. The redox pair of cobalt oxides Co3O4/CoO in particular, is characterized by high reaction enthalpies and thus potential heat storage capacity.Parametric testing of cobalt oxide-based powder compositions via Thermo-Gravimetric Analysis/Differential Scanning Calorimetry was performed to determine the temperature range for cyclic reduction–oxidation and optimize the process parameters for maximum reduction and re-oxidation extent. The heating/cooling rate is an important means to control the extent of the oxidation reaction which is slower than reduction. Complete re-oxidation was achieved within reasonable times by performing the two reactions at close temperatures and by controlling the heating/cooling rate. Under proper operating conditions Co3O4 powders exhibited long-term (30 cycles), complete and reproducible cyclic reduction/oxidation performance within the temperature range 800–1000°C. No benefits occurred by using Ni, Mg and Cu cobaltates instead of “pure” Co3O4. The Co3O4 raw material’s specific surface area is an influential factor on redox performance to which observed differences among powders from various sources could be attributed. Presence of Na was also shown to affect significantly the evolution of the products’ microstructure, though not necessarily combined with improved redox performance.

The electrohydraulic balance of the solar heat storage with autonomous power supply

The electrohydraulic balance of the solar heat storage with autonomous power supply

Structure and Performance of Calcined Bauxite

In order to further broaden the application field of calcined bauxite, using differential thermal analysis-thermo-gravimetry (DTA-TG), X-ray diffraction analysis (XRD), Scanning electron microscopy (SEM) and other testing technology to study and analysis on the high temperature performance of calcined bauxite in this paper. The results show that the main crystal phases of calcined bauxite are corundum and mullite, appearance is layered, granular distribution. Due to the thermal performance of calcined bauxite is optimal, and is expected to be used for the field of high temperature solar thermal heat storage material.

Heat Storage Performance Analysis of Solar Chimney Power Plant System

Solar chimney power plant which can realize continuous power generation is a potential power generation technology. Based on the heat transfer theory and hydrodynamics theory, a comprehensive heat transfer and flow model of solar chimney power plant was proposed, and the characteristics of unsteady temperature field and flow field of the system were analyzed. With Manzanares solar chimney power plant as the geometric prototype, the performance of plant generation output and generation stability were studied in condition of different thickness of heat storage layer, different specific heat capacity and thermal conductivity of heat storage medium. The numerical stimulation results showed that above three factors significantly affected the power generation output and generation stability. Thus, the optimization of the three factors was achieved in this paper, which provides the basis for the solar chimney heat storage system design.

Electric Power Generation from Solar Pond Using Combination of Thermosyphon and Thermoelectric Modules

Salinity-gradient solar pond is one type of solar collector with the ability to store thermal energy for long period of time and lower cost of construction compared with the other type of solar collector. It can collect and store solar heat at temperatures up to 80°C. A system in which heat from the lower zone is transferred to the hot surface of the thermoelectric modules using gravity- assisted heat pipes as thermosyphons has been investigated experimentally.The temperature difference between the lower convective zone and the upper convective zone is applied across the hot and cold surfaces of the thermoelectric modules. In the salinity gradient solar pond, a insulated solar pond with a surface area of 7 m2 and a depth of 1.3 m was built at Rajamangala University of Technology Isan Khon Kaen Campus,Thailand to conduct performance experiments. From the results of experiment by using water as working fluid, the temperature of the solar pond in lower convective zone is at 50°C.It can be seen that the thermoelectric is able to generate electricity at 36.25mV. Using R134a as a working fluid, the temperature of heat pond in lower convective zone is at 41°C.Due to this, thermoelectric generates electricity at 234.25mV.. Research results in the present work indicate that there is a significant potential for electric power generation from small solar ponds through a simple and passive device incorporating thermosyphons and thermoelectric cells.

Seasonal Performance of a Combined Solar, Heat Pump and Latent Heat Storage System

This paper investigates the seasonal performance of a combined solar, heat pump and ice storage system for residential dwellings. A simulation study is performed for seven different heat loads, i.e. building types, ranging from low-energy buildings to non-renovated existing buildings. Four reference buildings suitable for this heat generation system are presented and discussed in this paper. In addition, the solar ice system is applied to the three reference buildings defined in IEA HPP Annex 38/SHC Task 44 “Solar and heat pump systems”. The simulation results show that the solar ice storage system generates heat efficiently for a large variety of heat loads and consistently reaches a seasonal performance factor comparable to heat pump systems with borehole heat exchangers, i.e. SPF ∼ 4.

Simulation of a Vertical Ground Heat Exchanger as Low Temperature Heat Source for a Closed Adsorption Seasonal Storage of Solar Heat

This paper deals with the simulation of a vertical geothermal heat exchanger as low temperature heat source for a closed adsorption seasonal heat storage. The seasonal storage should allow reaching a nearly 100% solar fraction for space heating of a “low energy” building”. The selected adsorbent and adsorbate are respectively bromide strontium and water. The studied system, including the building and the ground exchanger, is simulated using the dynamic simulation software TRNSYS. Results show that expected performances are reached with a borehole of 100 m. The evaporation temperatures computed are really close to 0°C which might cause some problems. But an advanced research would maybe impose a deeper borehole to avoid cooling the ground on the long term.

Thermochemical Solar Energy Storage Via Redox Oxides: Materials and Reactor/Heat Exchanger Concepts

Thermochemical Storage of solar heat exploits the heat effects of reversible chemical reactions for the storage of solar energy. Among the possible reversible gas-solid chemical reactions, the utilization of a pair of redox reactions of multivalent solid oxides can be directly coupled to CSP plants employing air as the heat transfer fluid bypassing the need for a separate heat exchanger.The present work concerns the development of thermochemical storage systems based on such oxide-based redox materials and in particular on cobalt oxide; in the one hand by tailoring their heat storage/release capability and on the other hand via their incorporation in proper reactor/heat exchanger devices. In this respect the first stage of the work involved parametric testing of cobalt oxide compositions via Thermo-Gravimetric Analysis to comparatively investigate the temperature range for cyclic oxidation-reduction and optimize the cycle conditions for maximum reduction and re-oxidation extent. Subsequently, two reactor concepts for the coupling of solar energy to the redox reactions have been implemented and tested. These reactor concepts include in one hand structured ceramic reactors/heat exchangers based on redox-oxide-coated honeycombs and on the other hand powder-fed, solar-heated, rotary kiln reactors. The two reactor concepts were tested within non-solar-aided lab-scale and solar- aided campaigns, respectively. The feasibility of both concepts was shown and good chemical conversions were achieved. The experiments pointed out the challenging points related to the manufacture of pilot-scale reactors/heat exchangers with enhanced heat storage capacity. A numerical model using commercial CFD software is developed to define optimal geometrical characteristics and operating conditions and refine the pilot scale design in order to achieve efficient, long-term off-sun operation.

Thermochemical Seasonal Solar Heat Storage in Salt Hydrates for Residential Applications – Influence of the Water Vapor Pressure on the Desorption Kinetics of MgSO4.7H2O

An interesting thermochemical material for compact seasonal heat storage is magnesium sulfate heptahydrate MgSO4·7H2O. Previous studies in the field showed that this material presents a storage energy density of 1 GJ/m3 when the material is built in a TC storage system with a 50% porosity packed bed reactor. However, the material has slow reaction kinetics under the low vapor pressure typically occurring in a seasonal heat storage (13 mbar). The kinetic study presented in this paper shows that a higher water vapor pressure of 50 mbar increases the reaction kinetics of the dehydration process of MgSO4•7H2O, improving the performance of the material.

Development of a Closed Sorption Heat Storage Prototype

In order to overcome limitations of solar heating, great improvements in seasonal heat storage are required. Adequate heat storage is achieved by: reducing time dependent thermal losses, reducing storage volume, and allowing easy adjustment of storage geometries to enable building retrofitting.To this purpose much theoretical and practical work has been done at Empa, including a laboratory scale proof of concept of an aqueous sodium hydroxide based seasonal thermal storage.This storage concept is based on a continuous, but not full cycle, liquid state absorption heat pump. Heat is not directly stored; instead the potential to regain heat at a desired temperature from a low temperature thermal input is stored. The main benefit is that there are no capacity losses during storage time. For this reason there is great potential in the application of the closed sorption heat storage system for long term solar heat storage. Due to the losses encountered during charging/discharging (efficiency of the heat and mass exchanger), the concept is less suitable for short term heat storage. Therefore a hybrid system is proposed, consisting of hot water tanks for short term storage and closed sorption heat storage for seasonal storage.In the scope of the EU funded project COMTES a prototype aqueous sodium hydroxide seasonal thermal storage system is under construction. The system is dimensioned to cover space heating as well as domestic hot water in a single family house built to passive energy standards and located in the region of Zurich.

Modelling of Heat Exchangers Based on Thermochemical Material for Solar Heat Storage Systems

In solar thermal energy storage systems the operation modes involve charging and discharging. This paper focuses only on the charging leading to an endothermic reaction and therefore an efficient heat exchanger is required to transfer the heat for fast and complete charging. Two different heat exchangers are studied in this paper. A plate fin and helical coil heat exchangers embedded in a magnesium chloride bed is modelled and solved using the software Comsol 4.3a based on finite element method. Meshing analysis is performed for parameters sensibility and the results show a temperature variation of 13°C (helical coil) and 19°C (plate fin) in the material bed during the charging mode of the thermochemical heat storage system. The pressure distribution in the heat transfer fluid and the temperature distribution in the material bed are presented and the calculated overall heat transfer coefficient of 173W/m2·K (helical coil) and 236W/m2·K (plate-fin) are obtained on the base of the total heat transferred (Q) to through the system. The fluid flow is in turbulence regime (Re=13200) in the fin-plate, but in laminar mode (can be kept up to Re=20000) [1] in the coil because the flow is affected by secondary flow cause by centrifugal forces. This study allows the choice of the heat exchanger wherein with first experiment has been made and compared.