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Design of open volumetric receiver for air &supercritical CO2 Brayton cycle
- 1. Designof open volumetric receiver for air & supercritical CO2 Brayton cycle Usman Ali, M. Ahmad, H. Zahid Ali, Qamar Abbas 2014-UET-CCET-ME-05 2014-UET-CCET-ME-09 2014-UET-CCET-ME-14 2014-UET-CCET-ME-15 manimani7869@gmail.com muhammadahmad5101@gmail.com hafizzahidali622@gmail.com qamarabbas7824@gmail.com CHENAB COLLEGE OF ENGINEERING & TECHONLOGY GRW Department: Mechanical Engineering Submitted on 18 January 2017 Submitted To: Prof. Kamran Mahboob Abstract High-temperature receiver designs for solar powered supercritical CO2 Brayton cycles that can produce ~50 MW of Electricity is being investigated. Advantages of a supercritical CO2 closed-loop Brayton cycle with recuperation include high efficiency (~50%) and a small footprint relative to equivalent systems employing steam Rankine power cycles. Heating for the supercritical CO2 system occurs in a high-temperature solar receiver that can produce temperatures of at least 700 °C. Depending on whether the CO2 is heated directly or indirectly, the receiver may need to withstand pressures up to 20 MPa (200 bar). Designs for direct heating of CO2 include volumetric receivers and indirect volumetric receiver. Advantages and disadvantages of alternative designs are presented.
- 2. INTRODUCTION In solar power tower plants (SPTP) the receiver is a main component in which the concentrated solar energy is absorbed and then transferred to a working fluid. Volumetric receivers are characterized by having a highly porous absorber that allows solar radiation to be absorbed deep within its interior. A fan sucks the working gas (usually air) through absorbent pores, and the convective airflow takes up the heat. The outlet air temperature should be higher than the temperature of the absorber material on its irradiated side. Furthermore, the volumetric absorber design means that the effective area for solar absorption is many times larger than that of thermal radiation losses. The combination of these characteristics is termed the volumetric effect and results in a minimization of absorber thermal radiation loss. The volumetric concept has other promising characteristics, such as its simplified heat transfer mechanism, in which both solar absorption and convective heat transfer occur on the same surface. This reduces the material stress in the absorber and allows the outlet air temperature to approach its thermal resistance point. The large absorption surface permits high solar fluxes in the receive aperture, thus reducing the receiver size. Supercritical CO2 (s-CO2) thermodynamic cycles are projected to improve overall CSP plant economics due to their high power conversion efficiency and compact system layout. The s-CO2 Brayton cycle is essentially a recuperated, low pressure ratio closed cycle gas turbine concept. Use of a closed-cycle system permits a working fluid other than air, in this case carbon dioxide, and allows cycle operation at elevated pressures. For the s-CO2 Brayton cycle in particular, the high cycle thermal efficiency (45-50%) is achievable by means of pumping CO2 in its high pressure supercritical dense phase (at7.5MPa, 31°C), where little compression work is expended, then taking full advantage of recuperative heat transfer for preheating, before finally expanding the supercritical fluid at high temperature (at 20 MPa, 550-700°C). High fluid densities at the compressor and turbine (500 kg/m3 and 100 kg/m3, respectively) are largely responsible for an anticipated order of magnitude reduction in turbo-machinery size as compared to steam plants.
- 3. ALTERNATIVE RECEIVER DESIGNS 1.1. Direct Receiver Designs Direct receiver designs for s-CO2 applications require high pressures (up to 20 MPa) and temperatures (~500 °C – 700 °C). See figure 1 shows a schematic of a solar-driven, direct-heated, closed-loop s-CO2 Brayton power cycle. Several direct receiver designs are evaluated in the following sections. 1.1.1. Direct Volumetric Receiver Volumetric receivers use highly porous structures (honeycomb, foam, wire mesh, etc.) of metal or ceramic to absorb concentrated sunlight and allow penetration of the incident radiation into the volume of the receiver. This absorbed heat is transferred to a fluid passing through it. The thermal energy of the fluid is then converted to electricity by adopting suitable thermodynamic power cycles (e.g. Rankine, Brayton cycle, etc.) Directly heated volumetric receivers require a closed (pressurized) design that can be coupled directly to the gas turbine (single loop operation) to operate at higher pressures. Such receivers have been tested with absolute operating pressure of 15 bar and air outlet temperatures of 800 °C [9, 10] (see Figure 3). The higher
- 4. pressure operation is feasible because of a transparent window which separates the receiver cavity from ambient air. Also, the window minimizes thermal losses. However, containing s-CO2 at ~ 20 MPa (200 bar) and up to 700°C would require a robust window design having excellent optical properties to prevent reflective losses and degradation. It would need to be equipped with sealing and cooling systems to prevent mechanical damage and leakages. Other challenges associated with volumetric receivers are absorber durability and unstable gas flow, which, as predicted by Kribus et al. [11], can cause local overheating leading to its poor performance and local failures (melting or cracking). 1.2. Indirect Receiver Designs Indirect receiver designs for s-CO2 applications alleviate the need for high pressures within the receiver, but high temperature heat exchangers are required to transfer the heat from the heat-transfer fluid to the working fluid (s-CO2). Examples of heat-transfer fluid (media) include air, molten salt, liquid metals, or solid particles. The primary advantages of indirect receivers is the ability to incorporate thermal storage and to suppress transient effects (e.g., cloud passages). Figure 2 shows a schematic of a solar-driven, indirectly heated, closed loop s-CO2 Brayton power cycle that incorporates an additional heat exchanger and storage. Some heat-transfer media that can be stored directly include molten salt, liquid metals, and solid particles. Other heat-transfer media, such as air, would require and additional heat exchanger if storage is desired. Several
- 5. concepts for indirect s-CO2 receiver designs and heat-transfer media are presented in the following sections. 1.2.1. Indirect Volumetric Receiver Power plants with an indirect or open volumetric receiver design are generally equipped with two loops consisting of two different fluids. The heat-transfer fluid (usually air) at atmospheric pressure gets heated and pulled through the open volumetric receiver, and a heat exchanger transfers this heat to higher pressure fluid in the secondary loop. The secondary fluid then drives the turbine to generate power (e.g., 1.5 MWe Rankine cycle at the Julich Solar Tower, Germany). Brayton-cycle-based power plants consisting of s-CO2 as the working fluid would require an air-s-CO2 heat exchanger of large surface area due to the low overall heat transfer coefficient. The presence of two loops and high air temperature lead to higher thermal losses, thus increasing the system complexities and also reducing system efficiency.
- 6. 1.3. Summary Alternative high-temperature solar receiver designs for supercritical CO2 power cycles have been reviewed. Challenging aspects of implementing solar receivers with s- CO2 power cycles include high CO2 fluid pressures (~20 MPa) and temperatures (~500 °C – 700 °C). Receiver designs that can heat the s-CO2 directly include volumetric receivers and tubular receivers. Indirect receiver designs that require an additional heat exchanger to heat the s-CO2 from the heat transfer media in volumetric receivers. The heat-transfer media employed by indirect receiver designs can include air, molten salts, liquid metals, and solid particles. An inherent advantage of indirect receivers is the ability to store thermal energy for dispatchability of electricity as needed. Table 1 summarizes the benefits and challenges of each of the receiver designs reviewed.
- 7. RESIVER DESIGN Absorber, mixer assembly, and the air return cooling system (ARCS) are the major components of a volumetric air receiver. The absorber is a porous structure that transfers heat from solar radiation volumetrically to air with high solar-to-thermal efficiency. Despite non-uniform solar radiation flux incident on the absorbers, the mixer ensures uniform air temperature at the receiver exit. This will mitigate the resulting thermal stresses on the piping/joints/ connections to the rest of the system. Overheating of absorbers is prevented by the ARCS. In view of the selected application of solar convective heating and to fulfil its requirement, Open Volumetric Air Receiver (OVAR) would be more appropriate. A schematic diagram of an OVAR is shown in Fig. 3. The design of individual OVAR components will be discussed hereafter.
- 8. 2.1. Absorber Three major factors in the design of absorbers, namely, porosity, length of pores, and absorber material are considered. Clearly, for efficient volumetric heating, the porosity should be as high as possible subject to the mechanical integrity of the absorber. However, very high porosity would imply uniformly spaced smaller pore diameters, which may result in thermal instability. This is especially of special consideration for circular or similar types of holes. The absorber length should be such that the temperatures of the air and the solid are close to each other at the absorber exit. Thermal diffusivity of he absorber material should be sufficiently high to avoid high temperature gradients both in the axial and radial directions. 2.2. Mixer assembly The non-uniform solar radiation incident on the receiver would, in all probability, result in non-uniform temperature distribution in the air coming out from the seven absorbers resulting in thermal stresses on the receiver. Consequently, the mixer assembly homogenizes the air temperature coming out of the absorbers. The schematic of the OVAR including the designed mixer assembly is shown in Fig. 3, and it consists of (a) foot pieces connecting the outlet of each absorber to a mixing chamber and (b) a perforated plate with a convergent nozzle.
- 9. 2.3. Foot piece Fig. 4a shows a schematic diagram of the hollow, cylindro-conical foot piece attached to the rear of an absorber, the fabricated foot piece is shown in Fig. 4b. The main purpose of the foot piece is to homogenize the radial temperature gradient at the absorber exit. One end of the foot piece is inserted in the anchor plate, which secures the absorber attached to it (see Fig. 3) The design variables in a foot piece are (i) convergence angle and (ii) length of the cylindrical section, the optimum values of which were determined from CFD simulations. Analogy is drawn here from a venturi tube, which, to reduce undue drag, has entry and exit cone angles of 30 and 5 respectively. Therefore, convergence angles of 20 and 40 and a cylinder length of L = 25.4 mm were used in the simulations. For the performed CFD analysis, a mass flow rate of 0.00143 kg/s at the inlet of one foot piece. a foot piece with a convergence angle of 40 and 25.4 mm cylinder length was adopted as the final design. Perforated plate with convergent nozzle The purpose of the mixer assembly beyond the foot piece is twofold. One, it should generate sufficient turbulence to ensure a uniform temperature profile at the exit of the receiver assembly. Two, this system should generate sufficient pressure drop to circumvent flow instability in absorbers. Passive grids and perforated plates have been used for generating quasi-isotropic turbulence.
- 10. 2.4. Air return cooling system (ARCS) The hot air from the OVAR assembly transfers heat to a heat exchanger and/or thermal storage system. This relatively cooled air is recycled back into the OVAR for cooling and reducing the non-uniform temperature distribution in the absorbers and the related foot pieces, which, in turn, would mitigate the resulting thermal stress. Consequently, the ARCS also serves as a heat recovery system. The fraction of the returned or recycled air, which is sucked in after mixing with the atmospheric air is described as the Air Return Ratio (ARR). It is affected by the design of receiver and its material. The return air is injected mid-way between the absorber outlet and the anchor plate, as shown in Fig. 3. It then flows in the space between the absorbers and comes out from the front end of the receiver. As stated above, part of the return air is sucked back into the absorbers. Here the 6 circular inlets, 60_ apart, with air injection between foot pieces .
- 11. The space between the anchor plate and the perforated has a dual purpose: (i) preliminary mixing of the obtained hot air from foot pieces and (ii) collection and disposal of dust. This system would ensure that dust free air circulates in the OVAR and is being currently investigated. EXPERIMENTAL DESIGN OF OVAR The major objective of experimentally evaluating the OVAR was to check the validity of the calculation/assumptions made in designing different OVAR components.
- 13. Calculations: Components Details Casing Absorbers Footpiece Perforatedplate Mixer Return airinjection port Diameter of casing: 100 mm Totalno.:7,Diameterofpores:2mm,Totalno.ofpores:581,lengthofabsorber: 25.4mm,diameterofabsorber:25.4mm, porosity ~ 52% Convergent angle ~ 40°, length of the extruded portion: 25.4 mm Anchor plate, No. of pores in anchor plate: 7, diameter of pore: 19.5 mm Mixing chamber, Length of mixing chamber: 20mm Total no. of pores: 12, staggered arrangement, diameter of pore: 14.4 mm Convergent angle ~ 9.54°, length of mixer: 137 mm, length of extruded portion: 50 mm Placedmidwaytotheextrudedportionofthefootpieceandanchorplate.No.of injectionport:6,diameterofinjectionport: 11 mm Conclusions 1. A volumetric receiver intended for use in a s-CO2 Brayton cycle has been simulated using a numerical model using the Finite Element Analysis CFD FLUID FLOW. 2. The effect of mass flow rate and geometric parameters on the receiver efficiency and peak temperatures has been investigated. 3. The participation of s-CO2 in radiative heat transfer is an important factor in determining the peak temperatures in the receiver. 4. Design of a test loop that is intended to confirm the viability of the presented receiver concept and experimentally validate the CFD predictions is described.