Open Volumetric Air Receiver Research Articles

A Cost-Effective Open Volumetric Air Receiver Design Based on Free Floating Stackable Absorber Modules

The CAPTure Open Volumetric Air Receiver (OVAR) design is based on ceramic stackable “free floating” absorber modules that form the receiver structure and avoid a complex metallic double membrane design. The novel receiver design has been validated at small-scale (15 kWth) at a solar simulator, as well as on the top of an experimental tower at 300 kWth, up to absorber module outlet temperatures above 900ºC. The novel OVAR concept is attractive for both the concentrated solar power (CSP) sector, as well as for high-temperature process heat supply. The thermal efficiency of the receiver concept is expected to be above 80% at outlet temperatures of 900ºC.

A Multi-Zone Unsteady Heat Transfer Model for an Open Volumetric Air Receiver: A Step Towards Scale-Up and Design Optimization

A comprehensive, multi-zone, unsteady heat transfer model is developed for a straight absorber pore-based open volumetric air receiver. This includes heat exchange between the porous absorbers, absorbers and receiver casing, and absorbers and return air. The validation revealed its predictive capability within an uncertainty of ±7%. The model is used for the scale-up of receiver design with several intermediate absorber layers. The major recommendations for scale-up are a) multiple absorber layers is beneficial for mitigating thermal stress, b) higher flux concentration is required for non-volumetric heating, to achieve the desired air temperature, compared to volumetric heating, c) air return ratio should be 0.6, d) absorber porosity should be 0.6 for volumetric heating and higher for non-volumetric heating, e) absorber gap to length ratio should be 0.15 -0.25, f) the radiative heat loss is substantial for non-volumetric heating; therefore, the exposed surface area to ambient should be reduced, and g) absorber diameter to length ratio should be 1-2. The developed approach is generic and adaptable for the different open volumetric air receiver designs.

Open volumetric air receiver: An innovative application and a major challenge

AbstractThe open volumetric air receiver (OVAR) based systems can be used for metals processing operation, such as, the heat treatment of aluminum. Some important aspects, related to the technology adaptation, are (a) the selection of receiver design, (b) the integration of an application, based on hot air, with an OVAR, and (c) mitigation of dust deposition in the porous absorber of an OVAR. Therefore, the presented aspects are (a) literature review on the porous absorbers for OVARs, (b) literature review on solar furnace, and an innovative OVAR‐based solar convective furnace (SCF) for the heat treatment of aluminum, and (c) mechanism of transport and deposition of dust in an absorber pore. These reveal that (a) the design improvements are required, to enhance the thermal efficiency of OVARs, for an input power to air mass flow rate ratio exceeding 900 kJ/kg, (b) the SCF system, based on OVAR, is plausible, and (c) the dust deposition in an absorber pore may be mitigated. The findings will be useful for deployment and operation of the OVAR based systems in arid deserts.This article is categorized under: Solar Heating and Cooling > Science and Materials

On the dust deposition and its effects on heat transfer in absorber pores of an open volumetric air receiver

The deposition of dust in the absorber pores of an open volumetric air receiver (OVAR) offers a significant challenge in arid deserts. In this paper, to address this problem, two aspects are investigated, (i) the transport of an isolated dust particle in a straight, circular absorber pore, for a steady, thermally-hydraulically developing, laminar flow, and (ii) the consequences on heat transfer due to dust particles accumulation, in a straight, circular, absorber pore. The first problem is investigated with a semi-analytical approach and the second problem makes use of computational fluid dynamics simulations. The finding shows (a) a heat flux may be beneficial, for mitigating the dust deposition in a, straight, circular, clean absorber pore, (b) dust deposition in a straight, circular, absorber pore is detrimental to heat transfer, which may lead to its failure, (c) the thermal development length will be reduced for the dust deposited pore in comparison to a clean pore. The findings reveal a way of mitigating dust deposition in a clean pore, and the need for a cleaning strategy of dust deposited absorber pore, for sustainable operation of an open volumetric air receiver.

One-Dimensional Zonal Model for the Unsteady Heat Transfer Analysis in an Open Volumetric Air Receiver

Abstract The open volumetric air receiver (OVAR)-based central solar thermal systems provide air at a temperature > 1000 K. Such a receiver is comprised of porous absorbers, which are exposed to a high heat-flux > 800 Suns (1 Sun = 1 kW/m2). A reliable assessment of heat transfer in an OVAR is necessary to operate such a receiver under transient conditions. Based on a literature review, the need for developing a comprehensive, unsteady, heat transfer model is realized. In this paper, a seven-equations based, one-dimensional, zonal model is deduced. This includes heat transfer in porous absorber, primary-air, return-air, receiver casing, and their detailed interaction. The zonal model is validated with an inhouse experiment showing its predictive capability, for unsteady and steady conditions, within the reported uncertainty of ±7%. The validated model is used for investigating the effect of operating conditions and absorber geometry on the thermal performance of an absorber. Some of the salient observations are (a) the maximum absorber porosity of 70–90% may be preferred for non-volumetric and volumetric-heating conditions, (b) the minimum air-return ratio should be 0.7, and (c) the smallest gap to absorber-length ratio of 0.2 should suffice. Finally, suggestions are provided for extending the model.

A novel approach for modelling fluid flow and heat transfer in an Open Volumetric Air Receiver using ANSYS-FLUENT

Open volumetric air receiver (OVAR) has great potential as a source of process heat for metals processing operations. The two major components of OVAR are the porous absorbers and the return air flow chamber (RAFC). Heat exchange between the “cooler” return air and the hot curved absorber surface in RAFC is critical as it prevents overheating of absorbers. Unfortunately, modeling of air flow in the RAFC is conspicuous by its absence. Calculations were performed in ANSYS-FLUENT using the local thermal non-equilibrium model for heat transfer in the porous absorber. Three dimensional flow and heat transfer simulations were carried out in the RAFC. Unfortunately, ANSYS decoupled heat exchange between the absorbers and return air by splitting the curved absorber surface into real and imaginary surfaces. Decoupling also meant that heat flux had to be specified separately on the absorber and the RAFC, which was a challenge since only the total input solar flux is known. Hence the major contribution of this paper has been to incorporate two innovations in the simulation methodology. One, to couple the absorber and RAFC thermally by mapping the temperature of the real surface to the imaginary surface. Two, the flux used for heating air in the absorber was calculated iteratively. Limited simulations suggest that in laboratory experiments, relatively inexpensive circumferential (Joule) heating of absorbers can reasonably mimic heating of absorbers by concentrated solar radiation.

Novel hybridization of solar central receiver system with combined cycle power plant

This paper presents a novel hybridization of a solar central receiver system via an open volumetric air receiver with a combined cycle. The exhaust gases from a gas turbine before entering a heat recovery steam generator are heated up by the solar heat in a volumetric receiver working as a supplementary firing, in contrast to the solarized gas turbine where the solar heat is introduced into an open or closed Brayton cycle. This hybridization does not require significant modifications in the gas power block. In this study the thermodynamic and economic issues are investigated and complemented with an optimization to demonstrate its technical feasibility. The obtained results reveal noticeable enhancements in solar energy conversion and power plant performance, where the net solar energy ratio may reach up to 23.5% and the overall efficiency about 64.4%. Assessment of the levelized cost of energy led to an estimated value of 0.0335 $/kWh which could be further reduced through an optimization which when the environmental issue is considered becomes more competitive. Furthermore, there is an important fuel saving of 7.07 Million $ and less pollutant emission about 0.33 Million ton over 30 years of operating service.

The challenge of solar powered combined cycles – Providing dispatchability and increasing efficiency by integrating the open volumetric air receiver technology

This work analyzes the performance potential of solar-only powered combined cycles, comparing the impact of two different solar receiver technologies (opaque-heat-exchanger-type vs. volumetric). Due to material and receiver performance constraints, as well as the absence of internal combustion, the gas turbine inlet temperature (TIT) is limited to considerably lower values than observed in current fossil-fired state-of-the-art combined cycle plants. Therefore, the analysis includes the evaluation of a reheated topping Brayton cycle, aiming for a higher mean temperature of the heat input, thereby allowing fair conversion efficiencies despite moderate TITs. An extensive parametric optimization analysis compares different solar combined cycle configurations and benchmarks them against conventional CSP single-cycle plants. High thermal losses in the receiver tend to offset the gain allowed by the power cycle. The innovative coupling of an open volumetric air receiver with a regenerative heat exchange system that works in alternating operating modes (non-pressurized heating period, pressurized cooling period) could be a promising solution to efficiently drive a solar powered combined cycle. Furthermore, the optimum solar combined cycle performance for typical mean concentration ratios (C ≈ 500) is fully compatible with high temperature TES, providing the promising possibility of fully dispatchable operation at highest thermal-to-electric conversion efficiency.

Correction to: A Step Toward Realizing Open Volumetric Air Receiver Based Systems in Desert Regions

In the original publication, the format of Table 1 is Incorrect due to improper spanning of the rows. The correct table format is given below.

A Step Toward Realizing Open Volumetric Air Receiver Based Systems in Desert Regions

An open volumetric air receiver allows achieving a high temperature air for an innovative solar convective furnace system. Operating this receiver in arid-desert regions will pose a major challenge on account of the dust deposition in its porous absorbers. The resulting higher thermal resistance compared to a clean pore, even with a partial blockage, will lead to their failure. Therefore, a cleaning strategy based on the cyclone separator is already proposed for the receiver. Such an additional sub-system will reduce the performance of this air receiver due to the parasitic loss or pressure-drop. Considering these aspects, the paper presents (a) a correlation for the pressure-drop across a cylindrical absorber based on experiment and numerical simulation, (b) a limitation of a derived correlation for the thermal efficiency of an open volumetric air receiver based on experiment, and (c) an approach to determine the operating conditions of the receiver including a cyclone separator. The findings are expected to be useful for enabling concentrated solar thermal systems in arid deserts based on such receivers.

On the Flow Stability in a Circular Cylinder based Open Volumetric Air Receiver for Solar Convective Furnace

The arid desert regions are considered for installing the concentrated solar thermal based systems owing to the available direct normal irradiance. For process heat applications, like the solar convective furnace that requires an air temperature of 750 K the open volumetric air receiver is preferred. Such innovative concepts are envisaged for achieving high thermal energy conversion efficiency. For the prolonged operation of the solar convective furnace system the stable flow is necessary. On the other hand, the thermally induced flow instability is reported in the open volumetric air receivers at a high temperature. This may limit the operating temperature range of a receiver. In this paper the flow stability analysis in a circular cylinder based open volumetric air receiver with the straight pores is presented. The findings allow expecting a stable flow regime in such an absorber pore at a high flux concentration and temperature.

Reprint of: Dealing with dust – Some challenges and solutions for enabling solar energy in desert regions

In India, the desert regions of Rajasthan and Gujarat are preferred for installation of solar photovoltaic (PV) and concentrated solar thermal (CST) systems because of a high level of both DNI and GHI components of solar radiation. However, such regions pose substantial challenges in terms of dust, a high ambient temperature and scarcity of water. This paper is an attempt to address issues related to the deposition of dust (a) on a mirror with stand depicting PV panel or heliostat in the wake of a similar geometry and (b) in the porous absorber of an open volumetric air receiver (OVAR). A detailed insight to the wake-region behind an inclined flat-plate or heliostat is provided based on analysis and experiment. These indicate possible measures such as modification of the geometry of collectors to mitigate the problem of dust deposition. Also, the use of a cyclone separator with defined parameters based on a decision variable has been suggested for reliable operation of an OVAR. These problems being generic in nature, similar strategies are expected to be applicable in other desert regions subject to their specific conditions.

Coupled optical and CFD parametric analysis of an open volumetric air receiver of honeycomb type for central tower CSP plants

The performance of an open volumetric solar air receiver of honeycomb type (multiple parallel channels) for central tower CSP plants is evaluated numerically. A parametric study is conducted at the single channel level in order to investigate the influence of the main geometrical variables and of the air mass flow rate on the receiver performance. The adopted methodology consists of two steps: the first one is the optical analysis that is conducted using Tonatiuh, an open-source Monte-Carlo based ray-tracing software, providing the distribution of the absorbed heat flux on the channel inner surfaces, to be finally exploited as input data in the second step, i.e. the numerical evaluation of the thermal fluid dynamic performance of the channel. Using the commercial CFD software ANSYS Fluent, the convective heat transfer between the air flow and the absorber and the radiative heat transfer among the absorber inner walls and the channel aperture are simulated, computing the heat losses to the ambient. Different channel configurations are simulated, identifying the influence of the three key-parameters (the tilt angle with respect to the horizontal, the channel size and the air mass flow rate) on the receiver performance, in terms of solar-to-electricity efficiency.

Dealing with dust – Some challenges and solutions for enabling solar energy in desert regions

In India, the desert regions of Rajasthan and Gujarat are preferred for installation of solar photovoltaic (PV) and concentrated solar thermal (CST) systems because of a high level of both DNI and GHI components of solar radiation. However, such regions pose substantial challenges in terms of dust, a high ambient temperature and scarcity of water. This paper is an attempt to address issues related to the deposition of dust (a) on a mirror with stand depicting PV panel or heliostat in the wake of a similar geometry and (b) in the porous absorber of an open volumetric air receiver (OVAR). A detailed insight to the wake-region behind an inclined flat-plate or heliostat is provided based on analysis and experiment. These indicate possible measures such as modification of the geometry of collectors to mitigate the problem of dust deposition. Also, the use of a cyclone separator with defined parameters based on a decision variable has been suggested for reliable operation of an OVAR. These problems being generic in nature, similar strategies are expected to be applicable in other desert regions subject to their specific conditions.

On the evaluation of a cyclone separator for cleaning of open volumetric air receiver

Rajasthan and Gujarat on being located at the equatorial belt receive almost the highest solar irradiation in India. At the same time, such arid desert regions are blessed with dust. This energy can be harnessed using, for instance, solar thermal technology. The recovered heat can be employed for applications, like, heat treatment of metal. For this purpose, an open volumetric air receiver based solar tower technology concept is adopted. In this receiver concentrated solar irradiation is focused onto porous absorber, which is open to atmosphere. The atmospheric air is used as heat transfer fluid with suction. The suspended dust particles in atmosphere will deposit in the pores of the open absorber and even pass to the internals of the complete systems. This is expected to damage the receiver and prevent continuous operation of such a system. In this paper, design and evaluation of a cyclone separator for cleaning and collection strategy of dust from open volumetric air receiver is described. In particular, this paper presents (a) an experiment demonstrating the dust deposition in porous absorber (b) evaluation of the designed cyclone separator and (c) proposed modification in Lapple model for prediction of pressure-drop in cyclone separator including the effect of operating temperature.

Solar Convective Furnace for Metals Processing

Metals processing operations, primarily soaking, heat treatment, and melting of metals are energy-intensive processes using fossil fuels, either directly or indirectly as electricity, to operate furnaces at high temperatures. Use of concentrated solar energy as a source of heat could be a viable “green” option for industrial heat treatment furnaces. This paper introduces the concept of a solar convective furnace which utilizes hot air generated by an open volumetric air receiver (OVAR)-based solar tower technology. The potential for heating air above 1000°C exists. Air temperatures of 700°C have already been achieved in a 1.5-MWe volumetric air receiver demonstration plant. Efforts to retrofit an industrial aluminium soaking furnace for integration with a solar tower system are briefly described. The design and performance of an OVAR has been discussed. A strategy for designing a 1/15th-scale model of an industrial aluminium soaking furnace has been presented. Preliminary flow and thermal simulation results suggest the presence of recirculating flow in existing furnaces that could possibly result in non-uniform heating of the slabs. The multifarious uses of concentrated solar energy, for example in smelting, metals processing, and even fuel production, should enable it to overcome its cost disadvantage with respect to solar photovoltaics.

On the design and evaluation of open volumetric air receiver for process heat applications

India receives abundant radiant energy from the sun on account of being located at the equatorial solar belt. Especially, an annual global solar radiation of about ⩾2400kWh/m2 is received in Rajasthan and in northern Gujarat. As a part of research initiative at IIT Jodhpur, heliostat based concentrated solar tower using an open volumetric air receiver is being developed for generation of high-temperature process heat, which can be utilized for metal processing operations. This technology includes sub-systems such as thermal energy storage and heat exchangers. In particular, the presented paper describes the design evaluation of an open volumetric air receiver. Experiments and computational fluid dynamics tool, ANSYS-FLUENT are used for this purpose. The obtained hot air (heat transfer fluid) can be employed, for instance, in heat treatment of metal such as Aluminum. The considered design aspects of the open volumetric air receiver are as follows:a.Influence of material thermal conductivity on the heat transfer modelling and thermally induced flow instability;b.Thermal–hydraulic analysis of recirculating air injection system;c.Influence of absorber porosity on the heat transfer using the performed experiments;d.Design and evaluation of the designed mixer-assembly for uniform and non-uniform heating of porous absorber.In this paper, the first section presents order-of-magnitude analysis to quantify the effect of thermal conductivity of the solid on heat transfer process with porous absorbers. The second section deals with analysis of thermally induced flow instability in porous absorbers with the validated and numerically adopted computational fluid dynamics tool, namely, FLUENT using the reported experiment by Fend et al. (2004a). In addition, the effect of thermal conductivity on thermally induced flow instability is presented. The next section presents three-dimensional analysis of the designed mixer-assembly for receiver using the validated CFD tool FLUENT. For this purpose, analyses are performed with different designs of mixer-plate and for different receiver geometries. Further, based on the detailed analysis, the injection mechanisms of recirculating air for circular and square absorber based receivers are proposed. Finally, a 4kWth experimental facility, which is being commissioned at IIT Jodhpur to test solar thermal sub-systems such as receiver, heat exchanger, thermal energy storage and experimental evaluation of receiver components are described. These experiments demonstrate the effect of absorber-porosity, uniform and non-uniform concentrated solar irradiance level on heat transfer with porous absorbers and effectiveness of the selected mixer-assembly design in such a condition.

Open Volumetric Air Receiver Based Solar Convective Aluminum Heat Treatment Furnace System

Abstract The current heat treatment processes make use of electricity or fuel for generating heat. Solar energy is available in abundance in the states of Rajasthan and Gujarat in India. Annual global solar radiation of about ≥2400 kWh/m2 is received in these regions. Use of the abundant solar thermal energy in heat treatment of metals would saveenergy and fuels. In view of this a concept of solar convective furnace system is described in this paper. As a starting point, heat treatment of aluminium is considered. For this system, requirements of industrial furnace are taken as basis.A scale-down retrofitted furnace is designed and analysed.The importance of different process stages like, solar thermal energy absorption, storage and utilization in design of such a system is presented.A thermodynamic analysis is performed to derive requirements for the achievement of a uniform heating at a pre-determined rate. Control strategy to meet the requirements is worked out. Air flow profile in the furnace is analyzed using CFD as a tool. Laser Doppler Velocimetery technique is used for measurement of velocity in a Plexiglas model of the scale-down furnace. Flow analysis using theadopted CFD tool shows non uniformities in air flow profile. To counter this, further modifications and improvements in the furnace structure are suggested. Evaluation of Open Volumetric Air Receiver (OVAR) using installed solar air tower simulation facility is also presented in the paper.

Simulation of Operational Management for the Solar Thermal Test and Demonstration Power Plant Jülich Using Optimized Control Strategies of the Storage System

Electricity production costs and overall net efficiency of the system are determining factors regarding the quality and cost effectiveness of a solar tower power plant. These factors are strongly dependent on the quality of heliostats, the receiver and the thermal storage concept. This work describes the optimization of load states of a storage system through applying internal process control strategies for the air circuit of the Solar Tower Julich which works with open volumetric air receiver technology. A concept for optimal operation of the thermal storage system was formulated and developed. Storage concepts include both technical adjustments of the existing storage system as well as the control process of the thermal storage. For simulating the optimized operating strategy of the power plant systems all main power plant components were modeled in MATLAB®/Simulink® and interconnected to form a complete system. The focus of the simulation lies in optimizing the operating strategy of a discretized thermal energy storage model.

Dust Deposition Mechanism and Cleaning Strategy for Open Volumetric Air Receiver Based Solar Tower Sub-systems

Abstract Desert regions like Rajasthan and Gujarat in India receives abundant solar energy. At the same time these regions are blessed with dust or sand. Solar thermal systems are one of the ways to harness this available energy. Open Volumetric Air Receiver based concentrated solar tower systems are being investigated for applications, like, metal processing. The dust deposition on sub-systems like, heliostat, porous receiver will hinder smooth operation of such a system. Considering this fact, aspects of dust deposition in porous absorber of the receiver and in heliostat are presented. In this direction, experiments and analyses are performed. This revealed that dust deposition on heliostat will be affected by its location among other parameters. The presented analysis shows the required free-stream air velocity for cleaning of such a mirror depending on particle size and location. Experiments on dust deposition in a single pore of an absorber, simulated by a thin and long glass tube of 1.3 mm diameter is presented. Furthermore, experiments on dust deposition in one porous absorber reveal its severe consequence. Finally, a strategy for collection of the removed dust particles from these pores is presented to avoid their passage to internals.