Rock Bed Heat Research Articles

Experimental study on the heat storage characteristics of rock bed heat storage system with different packing structure

Establishing energy storage systems can provide an effective solution to the challenges posed by the variability and intermittency of renewable energy sources. Among the available options for energy storage, rock-packed bed thermal energy storage systems are widely favored for their performance, cost-effectiveness, and reliability. The more compact the rock packing, the more adequate the heat exchange of the system; however, poorer permeability increases fluid friction losses and pressure drop, which is detrimental to heat transport within the system. The permeability and heat exchange of the system jointly determines its thermal energy storage performance. This study proposes a composite packing scheme utilizing intact rock slabs and broken rock to enhance the thermal energy storage performance of the packed bed. This approach aims to augment heat exchange efficiency and elevate energy storage density while maintaining the bed's commendable permeability. Heat injection and heat extraction experiments of rock beds were conducted to study the heat storage characteristics of the rock-packed bed thermal storage system under different packing structure. In addition, this study investigates the effect of different injection pressures on the storage and release of heat in the system. The results show that the volume of the pore space of the filled bed, serving as the primary domain for gas flow and gas-rock heat exchange, is a decisive factor affecting the packed bed's permeability and heat exchange performance. When the porosity of the packed bed increased from 5.5 % to 9.9 %, its permeability increased from 1.42 × 10−13 m2 to 3.20 × 10−13 m2, yet the thermal exchange efficiency declined from 67.2 % to 65.3 %. Reasonably arranging intact rock slabs and broken rock fill can alter the path of heat transfer within the packed bed, thereby effectively enhancing its heat storage and release performance. For the same pore volume conditions, the heat exchange efficiency of the packed bed is increased by about 10 % by increasing the number of layers of broken rock fill. Additionally, high gas injection pressures can facilitate heat storage and release processes, but it does not mean that this is more efficient. The research findings can guide the selection of packing and injection schemes in packed bed thermal energy storage systems.

Experimental testing on the performance of solar dryer equipped with evacuated tube collector, rock bed heat storage and reflectors

Agricultural products can be dried in an open sun; but such drying method has poor quality of the product causes to grow bacteria and microorganisms and long drying time. In order to alleviate those draw backs of open sun drying, direct solar dryers were developed; but this type of dryer reduces the product quality due to direct exposure of solar radiation. Then, an indirect solar dryer became more important and efficient method. However, the most solar dryers were developed with less efficient flat plate solar collectors. A few solar dryers were developed using efficient solar collector called evacuated tube solar collector (ETC) but they have low overall performance. The goal of this research is to create a dependable solar dryer system for drying agricultural products. It was a solar dryer system with an independent rock bed thermal storage unit and an evacuated tube collector having reflectors. In this case, a rock bed has been a well-insulated with thermal insulation and its top surface is covered by a transparent double-glass. This system is intended to dry potato slice that weigh of 7.5 kg sample having a wet base initial moisture level of 80.1 %, until it reaches the ideal moisture content of 7–13 %. The solar dryer has been developed from drying chamber having size of (length x breadth x height) of 85 cm×61 cm x 62 cm, evacuated tube collector area of 1.3 m2, and rock bed volume of 0.137 m3 (with a solar absorber area of 0.52 m2 and a depth of 0.26 m). The solar dryer system was built and tested with loading slices at various air flow rates and open sun. Here, at a 0.016 m3/s air flow rate as compared to lower and higher air flow rates, more moisture reduction (9.3 % within eight hours and 0.664 kg/h), higher efficiency of the solar collector (62.02 %), more heat production from the system (20.79 MJ, of which 4837.8 kJ was recovered from the rock bed), and higher dryer efficiency (23.87 %) were attained. Compared to open-sun drying, 20 % less time was needed for drying. In addition, the study examined the significance of the reflectors and the rock bed. In comparison to the situation of utilizing ETC alone and 30 % of the drying period in relation to the open sun, the introduction of a reflector on the collector increased the average net heat obtained from the collector by 5.1 %. In another instance, the slice's moisture content was lowered to 12.37 % in under eight hours using ETC alone, while the dryer's efficiency was 29.82 %. However, the drying rate rise by 3.7 % in comparison to the dryer equipped with ETC alone because of the enhanced heat output in the dryer, which used ETC with a rock bed's help.

Performance study of low temperature air heated rock bed thermal energy storage system

AbstractOne of the primary types of sensible heat storage systems in drying applications is the packed rock bed. However, to create large‐scale heat storage systems for industrial use, one must comprehend the hydrodynamic and effectiveness of the heat transport mechanism inside the bed. In this study, the thermal storage unit uses river rock as heat storage materials with equivalent particle diameters of 36 mm in bed 1 and 56 mm in bed 2. The rocks were stacked in a truncated cone‐shaped concrete wall section with an average diameter and depth of 1.1 m and 1.3 m, respectively and a volume of 2.32 m3. During the charging phase, two airflow configurations were used, one from the top with an air mass flow rate of 0.753 and 0.332 kg/m2‐s and the other from the bottom with an air mass flow rate of 0.955 kg/m2‐s. During the discharging phase, the entire flow configuration is from the bottom section. It was observed that the mass flow rate and particle equivalent diameter had an important effect on the thermal performance and behaviour of the rock bed during charging and discharging operations. Maximum efficiency was achieved with an airflow configuration provided from the bottom when charging at 0.955 kg/m2.s. Consequently, a sizable quantity of heat or energy (60 MJ) was retained. It was also observed that the relationship between air mass flow rate and particle size was significant, with smaller particles retaining more energy. When comparing bed 1 with bed 2 at this air mass flow rate, bed 1 stored 2.1 times more energy than bed 2. A wind tunnel experiment was used to measure the pressure drop in the packed rock bed. The pressure drop in the bed increases with an increase in particle Reynolds number and decreases with an increase in particle size. Rock bed heat transfer coefficient and Nusselt number were calculated using the correlation that has already been established in the literature smaller particles showed higher heat transfer coefficients and lower Nusselt numbers. This is due to the increase in particle‐to‐particle interaction and larger particle surface areas. For a given Reynolds number, the Nusselt number increases with the size of the rock particle.

Potential for Solar Industrial Process Heat Systems for Tea Drying Applications – A Case Study

Abstract An increase in energy consumption especially for industrial applications enhances the uptake of fossil fuels since, they are currently the major sources of industrial process heat and electricity. The need for clean energy technologies is therefore critical, in order to meet the rising energy demand using sustainable energy sources. Solar thermal energy technologies have attracted much attention in terms of research and developmental activities worldwide. The present study seeks to explore the viability of adopting solar thermal technologies at Tingamira - Tanganda Tea estate, Zimbabwe. Data on current sources of heat, daily consumption of process heat and required process temperatures was gathered. Systems advisor model (SAM, version-2021.12.2) was applied to analyse feasibility of linear Fresnel (LF) and Parabolic trough (PT) solar thermal technologies for industrial tea drying. LF system is potentially a better option for producing process heat for low temperature applications. It requires smaller aperture area (360 m2) and a lower initial capital cost of $ 112 860 which is about 8 % cheaper. Also, it yields more energy at lower levelized cost of heat as compared to PT technology when solar multiple is increased up to 2. Replacing wood fuel would reduce the factory carbon footprint by 114 tonnes CO2 annually. Also, potential for sensible rock-bed heat storage systems in the tea factory was assessed. Rock bed sensible TES using natural rocks can avail a viable option for heat storage and about 16 m3 of storage volume would be needed to meet a daily heat demand of 1 MWh.

Numerical study on heat storage efficiency of broken rock mass under different CO2 injection conditions

The application of energy storage technology can solve the problems of randomness and volatility in the development and use of renewable energy, such as wind and solar energy, and effectively improve energy utilization. Rock heat storage is one of the primary forms of sensible heat storage. The heat storage efficiency and heat storage capacity of rock packed bed are important indicators to measure the energy storage effect of a rock energy storage system. This paper takes CO2 as the heat-carrying medium and broken granite grains as the packed bed matrix of the energy storage system. It establishes a porous medium thermal-hydraulic-mechanical coupling model of the broken rock bed. The heat injection and production process of the large-sized rock bed heat storage system is simulated using the COMSOL finite element software, and the heat storage efficiency and heat storage capacity of the broken rock bed is analyzed under different fluid injection pressures and injection temperatures. The results show that: 1) With the increase of CO2 injection pressure, the heat storage and heat production speed of the heat storage system will increase, and the heat storage rate changes from two stages of “low rise - slow decline” to four stages of “rapid increase—stability—rapid decrease—stability”; 2) With the increase of CO2 injection temperature, the maximum heat storage capacity of the rock bed heat storage system will increase. Due to the increase in the temperature gradient between CO2 and the rock bed, the heat storage and the heat production time of the heat storage system will increase, and its speed will decrease.

CFD Modeling of the Microclimate in a Greenhouse Using a Rock Bed Thermal Storage Heating System

The rock bed heating system is a more cost-effective concept for storing thermal energy use in greenhouses at night during the cold winter season. This system is considered an environmentally friendly solution compared to conventional heating systems that rely on fossil fuels. Despite the abundance of research on thermal energy-based heating systems, only limited work on climate modeling in greenhouses using rock bed heat storage systems has been reported. To fill this research gap, this study aims to simulate the microclimate in a greenhouse equipped with a rock bed heating system using computational fluid dynamics (CFD) models. User-defined functions have been implemented to account for the interactions between the plants and the air within the greenhouse. Crop rows and rock bed blocks have been considered as porous media with their dynamic and thermal proprieties. The model’s accuracy was approved by comparing simulated and experimental climate parameter data from the greenhouse. The model’s ability to predict temperature, humidity, and air velocity fields in the greenhouse as well as in the rock bed system during both phases of energy storage and restitution was demonstrated. The thermal, dynamic, and hygric fields were accurately replicated with this numerical model. The growing zone had a vertical temperature gradient between the ground and the greenhouse roof, as well as high humidity. The distribution of temperature fields along the rock bed blocks showed a significant temperature gradient between the air inlet and outlet in the blocks during the two phases of heat storage and restitution. As a result, the model could be useful for sensitivity studies to improve the performance of this thermal storage heating system.

Experimental study on heat exchange efficiency of rock bed heat storage system based on broken rock mass

Energy storage system can significantly improve energy utilization by ”cutting peak and filling valley” for the development and use of renewable energy sources such as wind and solar energy. Although rock packed bed is one of the main forms of sensible heat storage, understanding the variation law of its permeability and heat exchange efficiency is critical to the construction of large-scale heat storage systems. In this paper, we used CO2 as the heat transfer fluid in a self-built experimental equipment to conduct heat injection and extraction experiments on broken rock packed beds with different grain compositions and injection pressures, and we studied the heat exchange efficiency and the variation law of heat storage in the rock bed during the heat injection and extraction process. The results show that: (1) with the heat injection, the permeability of the rock bed filling system decreases gradually; and (2) the rock grain composition has little effect on heat exchange efficiency but a large effect on permeability. The higher the permeability, the greater the heat storage of rock bed; (3) The higher the fluid injection pressure, the greater the heat exchange efficiency and the greater the heat storage of the rock bed. The research results can provide guidance for the economic operation of broken rock bed heat storage system.

Experimental Study on Heat Exchange Efficiency of Rock Bed Heat Storage System Based on Broken Rock Mass

Experimental Study on Heat Exchange Efficiency of Rock Bed Heat Storage System Based on Broken Rock Mass

Experimental and modeling approach to heat and mass transfer in a porous bed of a rock-bed heat accumulator

The paper presents the results of the research concerning heat and mass transfer phenomena occurring in a porous bed of a rock-bed heat accumulator. The tests were carried out in a standardized semi-cylindrical foil tunnel, commonly used in horticulture farming, equipped with a rock-bed accumulator covering the area of 74.8 m2. Input data for the analysis were obtained from experimental studies carried out from April to October, including 435 charging and 409 discharging cycles. During the experiments, the process parameters, such as temperature, relative humidity and air flow velocity, in different measuring points of the system were recorded. Using modern mathematical modeling a multi-scale analysis was carried out to quantify the intensity of heat and mass transfer between the flowing air and bed particles, as well as the degree of humidification or drying of the air flowing out of the accumulator. The values of convective heat transfer coefficients and mass transfer coefficients for individual charging and discharging cycles were determined and correlation equations for them were found. The conducted analysis showed that the coefficients determined by measurements fit the proposed regression models with the following values of R2 (coefficient of determination): 0.83 and 0.86 for mass transfer coefficient (condensation and drying, respectively), and 0.85 for convective heat transfer coefficient. Using the similarity theory, the equations for calculating Nusselt and Sherwood numbers were found, from which the coefficients adopted for analysis can be derived. It was also analyzed to what extent the obtained results differ from the results presented by other authors describing similar issues. The originality of this research consists in determining the values of heat and mass transfer coefficients for the air flow through the porous bed and finding their functional dependencies on the input parameters. The research results fill the gaps in scientific knowledge; moreover, they can be applied in heat transfer engineering issues, such as modeling heat and mass processes occurring in a rock-bed accumulator in a greenhouse or in other facilities, thus enhancing the energy efficiency of the designed systems.

Thermodynamic modeling of a solar energy based combined cycle with rock bed heat storage system

In the current study, a combined system which consists of gas and steam turbines supported by concentrating solar receiver is studied through a thermodynamic approach using energy and exergy methods. Because of the intermittent nature of solar energy, a rock bed thermal energy storage system is incorporated into the system. The rock bed storage allows the secondary cycle to operate independently in order to generate electricity at night or once the solar energy is not available. Charging capacity of the rock bed is 9 MW for an 8 h period of time. The rock bed stores thermal energy for 4 h and thereafter, discharges it to run the steam cycle for 12 h. The energy and exergy efficiencies of the system are investigated through specific parametric studies to examine the effects of changing state properties and working conditions on energy and exergy efficiencies. According to the performance evaluation, the gas turbine could generate 3.87 MWe power while 1.76 MWe power is generated by the steam turbine. The overall energy and exergy efficiencies are obtained to be 69.2% and 37.3%, respectively.

A Preliminary Study on Rock Bed Heat Storage from Biomass Combustion for Rice Drying

One of the main constraints of biomass fuel utilization in a small scale rice drying system is the operating difficulties related to the adjustment of combustion/feeding rate. Use of thermal storage may reduce the problem since combustion operation can be accomplished in a much shorter time and then the use of heat can be regulated by simply adjusting the air flow. An integrated biomass furnace-rock bed thermal storage with a storage volume of 540 L was designed and tested. There were four experiments conducted in this study. Charging was performed within 1-2 hours with a combustion rate of 11.5-15.5 kg/h. In discharging process, the mixing of air passing through the rock bed and ambient air were regulated by valves. Without adjusting the valve during the discharging process, air temperature increased up to 80°C, which is not suitable for rice batch drying process. Charging with sufficiently high combustion rate (14 kg/h) within 1 hour continued by adjusting the valve during discharging process below 60°C increased the discharge-charge time ratio (DCTR) up to 5.33 at average air temperature of 49°C and ambient temperature of 33°C.The efficiency of heat discharging was ranged from 34.5 to 45.8%. From the simulation, as much as 156.8-268.8 kg of rice was able to be dried by the discharging conditions.

Experimental investigation on heat extraction from a rock bed heat storage system for high temperature applications

Solar energy is available in an intermittent way, and integrating an energy storage system with solar energy collection devices may promote uninterrupted supply of energy in the absence of the availability of solar energy. It has been shown that heat can be stored using rocks packed in a bed, but limited work has been reported on heat extraction from a charged rockbed. This paper reports on the heat extraction from a charged rock bed. Discharging tests were performed under different air flow conditions and initial bed temperatures. Without the blower, the discharging rate is very slow. The discharging rate can be increased, and the cooking time controlled by adjusting the air speed through the rock-bed system.

Dynamic model of a small scale concentrating solar cooker with rock bed heat storage

This study presents a dynamic model for a concentrating solar energy collector with an integrated rock bed heat storage system. The model is based on numerical integration of a set of conservation equations for mass, momentum and energy of the heat carrier, the rock pebbles and the walls. The heat carrier is compressible air. Numerical solutions are implemented based on implicit time integration without iterations. Stability problems at large time steps do not occur but the accuracy is reduced. The model predicts pressure, velocity, density and temperatures of the fluid, rock bed and wall in time and along the bed. The model is validated with experimental results in a laboratory setting on temperature profiles during charging and discharging of rock bed heat storage. The intention is that the model shall serve as a computational tool for upscaling of air based concentrating solar energy systems with rock bed heat storage units.

Modeling of energy performance of a house with three configurations of building-integrated photovoltaic/thermal systems

A theoretical and experimental study of energy performance of three different open loop air heating building-integrated photovoltaic/thermal (BIPV/T) systems that utilize recovered heat for home heating is presented. The configurations are: Configuration 1: base case of unglazed BIPV with airflow under it; Configuration 2: addition of 1.5 m vertical glazed solar air collector in series with Configuration 1; Configuration 3: addition of a glazing over the PV. The model developed has been verified against experimental data from a solar research house for Configuration 1. Obtained relationships for BIPV/T system exiting air temperature as function of solar irradiance and air speed in PV cavity may be used for developing fan airflow control strategies to achieve desired outlet air temperature suitable for different applications. For Configuration 1, preheated air is suitable for HVAC system and domestic hot water (DHW) preheating. Higher outlet air temperatures of the PV cavity suitable for DHW might be achieved by utilizing Configurations 2 or 3. With Configuration 2, significant outlet air temperatures are achieved in winter along with enhanced thermal efficiency making it suitable for coupling with a rockbed heat storage. Finally, Configuration 3 significantly reduces electricity production and may lead to excessively high PV panel temperatures.

96/00563 Integrated rock bed heat exchanger-cum-storage unit for residential-cum-water heating

96/00563 Integrated rock bed heat exchanger-cum-storage unit for residential-cum-water heating

Integrated rock bed heat exchanger-cum-storage unit for residential-cum-water heating

This paper describes the results of a simulation study of a forced circulation, solar hybrid residential-cum-water heating system which comprises a corrugated absorber water heater and a rock-bed water-to-air heat exchanger-cum-storage unit integrated to a residential building to be heated. The system has been evaluated without and with the hot water load (which is the standard hot water demand for a typical family of two adults and two children in India) in the residential building. To satisfy the heat demand, no auxiliary energy has been assumed to be supplied to the system. The rate of air flow in the system was observed to influence the system performance most significantly. In addition, the mode of operation, i.e. whether the air flow is continuous or intermittent, influences the system performance greatly, a higher and intermittent air flow rate resulting in a more stable and uniform temperature of the living space.

Dynamic simulation of radon daughter concentrations in apartments using solar rockbed heat storage

In solar rockbed storage systems, heat is transferred during the day from the collector to a bed of pebbles, and released at night to warm the living space. When the rocks used for storage contain significant concentrations of uranium, 222Rn and its daughters may be released to the living area. A microcomputer model was used to simulate variations in air filtration rate and source strength through several days of operation. Source strengths were estimated from theoretical considerations and literature data. Resulting 222Rn and daughter concentrations were computed by solving system equations by fourth-order Runge-Kutta integration. During the day, when the living space is isolated from the radon source, interior 222Rn concentrations approach those of the outdoors. A nighttime steady-state concentration is approached about 6 h after heat discharge begins. Due to the dynamic nature of the simulation, equilibrium between 222Rn and its daughters is not reached. Time-weighted average nighttime exposures (6 p.m.–8 a.m.) for 10 simulation runs varied from 0.001 to 0.018 working level (WL). Comparison with one set of measurement data showed the model to overpredict concentrations but to approximate the 222Rn buildup rate well. Combinations of source strength, infiltration rate, and exterior radon concentration which would lead to exposures exceeding 0.02 WL were calculated.

Pressure Drop and Heat Transfer in Crushed Limestone

ABSTRACT PRESSURE drop due to air flow through crushed limestone, which has been screened to a rather uniform size and compacted into a container, has been correlated with data available in the literature. However, the pressure drop for uncompacted samples and for varied sizes of stone within the same sample, could not be predicted using any of the available data in the literature. Analytical and experimental studies of heat transfer between the flowing air and crushed limestone show that for reverse flow discharge the quantity of heat which can be recovered from a rock bed decreases as rock size increases. The suggested design procesure for rock bed heat storages utilizing reverse flow discharge is to select the smallest size stone which is consistent with the maximum allowable pressure drop through the bed.