TITLE: "Three Stage Parabolic Dish Collector cum Receiver" INVENTOR/APPLICANT: 'Santosh 5. Bopche'
The following specifications particularly describe the invention and the manner in which it is to be performed.
FIELD OF THE INVENTION
[0001] The present invention is related to the solar radiation concentrating at three focus
points, at which three stages (low, medium and high temperature) of receiver are located. It refers to the improvement of receiver thermal collection efficiency by using three staged receiver. It discloses the device design that concentrates the incident solar energy in increasing proportion subsequently at low, medium and high temperature solar cavity receiver stages.
BACKGROUND OF THE INVENTION
[0002] The solar energy is used in limited sectors of applications due to its dilute
distribution over the earth surface and its dependency on seasonal variations. The solar energy can be concentrated either by using plane mirror reflectors, parabolic dish collectors, compound parabolic dish collectors or by using Fresnel lens. The use of Fresnel (concentrating) lens makes the system relatively expensive.
[0003] A solar concentrator acts as a solar radiation to reflect and converge solar
radiation at a heat transferring device, called as receiver. A receiver serves the purpose of transferring concentrated solar energy to the working fluid. The heated or phase changed working medium is used to drive the prime mover which ultimately supplies to the electric generator for electricity generation. A Stirling or Brayton engine can be used to serve this purpose.
[0004] A parabolic dish collector intercepts the incident solar radiation and reflects it at a
fixed location, focus, the distance of which from the vertex of collector depends on collector design. The amount of solar energy concentrated at the focal location depends on the

concentration ratio which is defined as the ratio of area of collector opening to the projected area of absorber or receiver opening. The absorber is a black surface exposed to the incident solar radiation may be a tube or a spherical surface enclosed in a glass cover. The insulation is provided over the hot metallic surfaces or vacuum is maintained between the glass cover and absorber surface to reduce down the energy losses. For achieving high temperature at absorber tube or receiver, highly concentration device is preferred.
SUMMARY OF THE INVENTION
[0005] A higher receiver-wall temperature causes temperature dependent radiation
emission as well as conduction-convection losses. Minimizing these losses is vital for realization of higher efficiency systems through high temperature. Splitting the receiver aperture into detached stages according to incident radiation distribution significantly reduces energy losses from the receivers acting as a mere collection device. The working fluid gains energy as well as temperature gradually as it passes through a sequence of receiver stages. Only the high temperature stage receiver wall remains at a very high temperature that minimizes the overall thermal loss from the claimed receiver.
[0006] The low temperature stage is of hemispherical shape, a cavity with partly closed
circular opening, accepts concentrated energy from centrally located small sized parabolic dish at lower concentration. The second stage receiver bottom end is of hemispherical shape while the top one is of conical shape, traps energy re-reflected from the middle collector dish at medium concentration. The third stage is conically annular enclosure type, the enlarged opening of which receives highly concentrated energy via outer parabolic dish. The parabolic collector dishes are expanded accordingly.
[0007] The low temperature receiver stage (stage i) 17 is enclosed in an intermediate
temperature stage 20 so as to reduce down energy losses from the top of the receiver wall. The intermediate temperature stage (stage II) is enclosing inside the higher temperature stage 22 of receiver. The top wall of higher temperature stage (stage III) of receiver is insulated by using glass wool insulation 23. This is how the energy losses are minimized by shading bottom wall of higher temperature stage receiver using intermediate temperature receiver wall while the bottom

wall of intermediate receiver stage is shaded by lower temperature stage. The top wall is insulated sufficiently; the bottom stage is kept opened to trap energy concentrated and reflected from low temperature dish 1 and the chances of energy loss through the annular end openings of medium and high temperature receiver stages are minimized by providing glass covers 16, 21, 29.
[0008] Multistaging of collector cum receiver system reduces the installation costs of the
larger collecting areas that are generally used nowadays for power generation. One such three-stage-collection system nullifies the additional cost of installation for two additional concentrators and additional two receivers.
DESCRIPTION OF THE APPARATUS DRAWINGS
[0009] FIG 1-Figure 1 illustrates front perspective view of solar parabolic dishes with
increasing focus point distances; bottommost smaller dish 1 is having a closer focus point represented by distance 7 from the vertex of dish 10. The respective focal distances for the intermediate and higher temperature dishes are represented by 8 and 9. The low temperature dish 1, intermediate temperature dish 3 and higher temperature dish 5 have focus points situated at locations 2, 4 and 6, respectively.
[0010] FIG 2-Figure 2 demonstrates receiver stages operating at lower, medium and
higher ranges of solar concentration ratios and accordingly the wall temperatures or temperatures of working fluids. Lower temperature stage of receiver 12 is hemispherical in shape with partly closed circular opening 16 through which concentrated solar energy focuses inside the cavity 17. The working fluid (water) flows through the circular tubes 13 made of stainless steel are spirally made integral with the cavity walls (low temperature stage I receiver wall 12, bottommost medium temperature stage wall 14, topmost conical wall 15 of medium temperature stage receiver, bottom wall 18 of higher temperature stage receiver & top hemispherical wall 19 of higher temperature stage receiver).
[0011] The stage-I 17 is low fluid temperature cavity receiver, the stage-II is medium
fluid temperature stage receiver 20 and the stage-III 22 is higher fluid temperature stage of

receiver. The concentrated energy reflecting from medium temperature parabolic dish 3 enters inside the stage-II of receiver through annular opening 21. Similarly, highly concentrated energy coming from largely expanded parabolic dish 5 focuses inside the cavity 22 through annular opening 29.
[0012] The intermediate stage-II of receiver is having bottom wall 14 of hemispherical
shape and the topmost wall 15 of conical shape. However, the higher stage-Ill of receiver is having bottom wall 18 of conical shape and top wall 19 is of spherical domed shape. The top wall of three staged receiver is insulated sufficiently by using Glass Wool 23 of thickness maximum possible to reduce energy losses. Such insulation can be provided within the gap between receiver walls 12 and 14, also between the walls 15 and 18.
[0013] FIG 3- The assembly of three parabolic dishes 24 and three stages of receivers;
26 (low-temperature stage), 27 (medium-temperature stage) and 28 (Higher-temperature stage) are illustrated in Figure. 3. The assembly of parabolic dishes 24 are designed in such a way that the concentrated incoming solar irradiance 11 merge at focal locations 2, 4 and 6 situated deep inside the stage 1, stage 2 and stage 3 of three stage receiver 25.
COMPLETE DESIGN SPECIFICATIONS
[0014] The three staged parabolic dish collector cum receiver assembly have been
designed for an electrical power rating of 5 kW and working fluid pressure of 5 bar. Assuming overall energy conversion efficiency as 10 to 15 % the thermal power to be collected by the receiver is taken, about 33.33 to 50 kW.
[0015] The volume flow rate of working fluid (water/steam) needed for electrical power
production of 5 kW is obtained as 5 to 10 LPM (liter per minute equivalent to approximately 0.1 kg/sec). The diameter of working fluid circulating tube is d = 0.5 Inch ~ 0.012 m. The obtained values of fluid flow Reynolds number is 23951, Prandtl Number for two phase flow of water is 1.165 and the heat transfer coefficient (for forced convection) is 455 W/m K.

Design of Stage I (Collector cum Receiver System), Focus (150 cm)
[0016] Doing energy balance before and after concentrating the incident solar radiation

The heat flux available at receiver aperture is given by,

For a concentration ratio of stage

[0017] Energy available at the receiver opening of

4783 W. Assuming, uniform distribution of energy available at the hemispherical cavity receiver and with a tube side heat transfer coefficient of 455 W/m K, the surface area required for attaining a working fluid temperature above 100°C, is obtained as follows; (Tw is the wall temperature and Ttuik-jiuid is the bulk fluid temperature).

Required heat transfer area for absorbing 4783 W energy = 0.216745 m2
The diameter of hemispherical cavity receiver is obtained as 39.89 cm. The temperature of working fluid at the exit of receiver Stage-I (hemispherical shape) is equal to 110.53 °C.
[0018] The maximum wall temperature of receiver stage I is equal to 291.5°C. For a
receiver opening diameter of 20 cm and the focus location for stage I fixed at 150 cm from the vertex of parabolic dish stage-I, the diameter of hemispherical modified cavity receiver is obtained as 39.89 cm.

Design of Stage II (Collector cum Receiver System), Focus (240 cm)
[0019] Pursuing, energy balance before and after concentration of incident solar radiation
and doing energy balance before and after concentration of incident solar radiation

Concentration ratio of stage

128, the heat flux at the stage II receiver is


obtained. as, qreceiver aperture

[0020] Energy available at the receiver annular opening of size 0.35 to 0.2 m
is 8293.8 W. The dimensions of combined hemispherical-conical shaped (stage II) receiver as (Radius of hemisphere, 0.2 m; radius of cone, 0.175 m and height of cone is 0.24 rn). The heat transfer area of stage II receiver is obtained as 0.3228 m . The temperature of working fluid at the exit of receiver Stage II is 272.2 °C and at inlet is 110.5 °C. The maximum wall temperature of Receiver Stage II is 416.4°C
Design of Stage III (Collector cum Receiver System), Focus (310 cm)
[0021] From, energy balance before and after concentration of incident solar radiation
and doing energy balance before and after concentration of incident solar radiation

Concentration ratio of stage

the heat flux at the stage II receiver is


obtained as, qreceiver aperture

[0022] Energy available at the receiver opening of size 0.38 to 0.22 m is 10555.75 W.
Assuming, uniform distribution of energy available at the modified hemispherical cavity receiver and with a tube side heat transfer coefficient of 455 W/m K, the surface area required for attaining a working fluid temperature above 500°C, is obtained as follows. The heat tranfer area necessary for absorbing 10555.75 W of energy is 0.4639 m2.
[0023] The dimensions of Receiver Stage III is obtained as radius of outer hemisphere,
0.19 m; radius of cone, 0.11 m and height of cone is 0.1 m. It is selected with reference to the arrangement of parabolic dishes and focus distance of stage III that is 310 cm from vertex of dish collector. The temperature of working fluid at the exit of receiver Stage III is 500 °C and at inlet is 272.2 °C. The maximum wall temperature of Receiver Stage III is about 630.96°C.
SUMMARY OF COMPLETE SPECIFICATIONS
[0024] The required Steam temperature at a mass flow rate of 0.1 kg/sec (at 5 bar pressure) is equal to 450°C. The estimated designed value of steam temperature at outlet of stage III of receiver system is around 500°C.
[0025] For a maximum collector stage I diameter of 2.5 m, the temperature of working fluid obtained at the outlet of receiver stage I (hemispherical shaped) is 110°C (approximately). Similarly for a collector dish size (stage II) of diameters (outer 4.1 m and inner 2.5 m\ the temperature of steam at outlet of receiver stage II (Combined hemispherical and conical) would be approximately equal to 272.2°C. For a dish collector of stage III diameters (dish outer diameter of 5.5 m and inner of 4.1 m, the temperature of working fluid obtained at the outlet of receiver stage III (combined conical and hemispherical) is 500°C which is sufficient enough for electricity generation of 5 kW through the Heat Engine device.

CLAIMS
1. I claim a novel design of coaxial three staged parabolic dish collector and three staged receiver in present invention. The present creation of three stage parabolic dish collector 24 (Figure 3) focuses incoming solar radiation at three stages of receiver e.g., low temperature stage 26, medium temperature stage 27 and high temperature stage 28. The gradual heating of working fluids while passing through the three-staged-receiver wall minimizes the energy losses to the extent that leads to improvement in solar receiver collection efficiency.
2. Claim 1 comprises coaxially arranged three parabolic dish collectors all are receiving incoming solar radiation and focusing it concentrated at three aligned focal points exactly situated inside the three stages of receivers. Out of three dish collectors two outer collectors are truncated one. Each parabolic dish has different focus location.
J. Claim 1 also comprises; hemispherical shaped cavity type low temperature stage I 26 of receiver and combined hemispherical-conical shaped cavity type medium 27 and high temperature stage 28 receivers.