WO2006035455A2

WO2006035455A2 - Solar flat plate fluid heating device

Info

Publication number: WO2006035455A2
Application number: PCT/IN2005/000311
Authority: WO
Inventors: Milind V. Rane
Original assignee: Rane Milind V
Priority date: 2004-09-14
Filing date: 2005-09-13
Publication date: 2006-04-06
Also published as: WO2006035455A3; WO2006035455B1

Abstract

The present invention relates to a novel high efficiency and low weight solar flat plate fluid heating device. Use of a translucent multi-wall top cover (14) with plurality of integrated conduits in combination with stepped multi-wall sheet absorber (21) results in reduced top losses while reducing weight of the device and fluid pressure drop. Blower/pump power reduces substantially enabling the use of PV operated blower/pump. Preheating of fluid entering the absorber is facilitated by the use of translucent multi-wall top cover (14). Multi-wall construction of the absorber (21) results in reduced hydraulic diameter thereby enhanced heat transfer. Further, stepped and/or parallel arrangement of the absorber (21) results in reduced fluid pressure drop. Material of the multi-wall sheet is selected from plastics, polymers, glass, metals, alloys etc. The novel device is capable of heating single or plurality of fluids simultaneously. Further, it can be effectively used at low ambient temperatures of the order of -20° C.

Description

SOLAR FLAT PLATE FLUID HEATING DEVICE

Field of the Invention

The present invention relates to a novel high efficiency and low weight solar flat plate fluid heating device.

Background of the Invention

Flat plate collectors are used to heat fluids such as air, water, etc. A conventional flat plate solar air heater essentially consists of an absorber plate, a transparent cover system on the top, and insulation on the bottom and sides. The assembly is encased in sheet metal.

Heat loss from the top surface of the collector which is exposed to solar radiation is known as top loss. It primarily consists of heat loss resulting from conduction, convention and radiation from the top surface of the absorber to ambient thus reducing efficiency.

Conventional air heaters use sheet of glass (around 3 mm thick) as top cover which results in increasing weight of the solar heater. It may be noted that a single glass sheet is incapable of reducing top losses thereby requiring plurality of glass sheets need to be used to substantially reduce the top losses. Thus, conventional solar air heaters suffer from disadvantages of being heavy and at the same time exhibiting substantial top losses and reduced efficiency.

In jet plate solar air heater suggested by Choudhary and Garg (C. Choudhary and HP. Garg, Solar Energy, 1991) and US Patent: 4085729, an additional plate with a number of holes used between absorber and the bottom plate. The air entering the heater flows in between the absorber plate and the jet plate (flow m₂) as well as between the jet plate and the bottom plate (flow m-i). The flow m-i impinges out of the holes and hits the bottom of the absorber plate before mixing with flow m₂. This construction provides for higher turbulence at the cost of higher undesirable pressure drop. Further, glass sheet as top cover is unable to arrest and reduce top losses substantially; this reduces efficiency of the device.

US Patent number 6082354 discloses a solar collector with a radiation-absorbing member constructed of a light, low-cost material, without problems associated with fluid pressure inside the member, and with a satisfactory efficiency. Solar radiation absorbing plate is used; it consists of two substantially parallel plastic layers which are interconnected by means of transverse walls of the same material, preferably polycarbonate. However, since top cover is not used in this device, the top losses are high resulting in lowering the efficiency.

M.S. Sodha and A. Kumar, 1984 describe overlapped glass plate air heater consisting of a set of glass plates kept one over the other. The portion of the glass plate which is behind the preceding one is blackened while the rest is transparent. Due to solar radiation, the black portions of the glass plates are heated and air flowing parallel to these plates gets heated. Advantages of such a heater include a low pressure drop and high efficiency for moderate temperature rises. However, use of multiple glass plates substantially increases weight of the air heater. Since hydraulic diameter of the passages for fluid is high, heat transfer efficiencies are low.

Mc Cullough, 1982, US Patent: 4334524 disclose air heaters with honeycomb absorber. Honeycomb structure is used between the cover and the absorber to reduce top convective losses. The honeycomb structure consists of cubic or hexagonal cellular structure and is made from a transparent material like glass or plastic (thermoformable aterial). The honeycomb structure may be in firm mechanical contact with the cover here it acts as a air buffer layer reducing convective and radiative losses. Air can be passed through honeycomb structure along with the porous absorber. However such system suffers from shortcomings such as is the presence of adhesive bonds between adjacent cells, acting as scattering sites for incident radiation resulting in reduction of effective solar intensity incident on the absorber. Aging of adhesive bonds result in reduced collector efficiency due to embrittlement, cracking and discoloration.

Collectors used for water heating using multi-wall absorber with and without top covers are reported in the literature. Since single cover is utilized, top loss is not reduced significantly. This results in reduced efficiency of the collector.

A study of the prior art related to flat plate solar heating devices reveals following technological gaps:

• Use of a single glass sheet is incapable of reducing top loss, resulting in reduced efficiency of the fluid heating device.

• Multiple top covers used to reduce top loss result in increase in the weight and cost of the device

• Jet plate solar heaters are subjected to significant fluid pressure drop and result in increased blower/pump power

There has been a longstanding need to provide a flat plate solar fluid heating device featuring:

• Reduced top losses while reducing weight of the device

• Enhanced efficiency • Reduced fluid pressure drop

• Reduced weight

• Improved cost effectiveness

Summary of the Invention

The main object of the invention is to provide a novel solar flat plate fluid heating device with reduced top losses, enhanced efficiency and reduced weight. Yet another object of the invention is to substantially reduce fluid pressure drop while enhancing heat transfer resulting in enhanced efficiency.

Yet another object of the invention is to heat two fluids simultaneously in the same device.

Thus in accordance with the invention the flat plate solar fluid heating device comprises of

• Translucent multi-wall top cover with plurality of integrated flutes/passages/cavities

• Opaque multi-wall absorber with plurality of integrated flutes/passages/cavities

• Inlet and outlet fluid conduits for absorber • Optional inlet and outlet conduits for the top cover

• Optional blower/pumping means

• Insulation provision

• Optional reflecting means placed under the said absorber

DETAILED DESCRIPTION OF THE INVENTION

Features and advantages of the invention will become apparent in the following detailed description and preferred embodiments with reference to the accompanying drawings:

Figure 1 Constructional features of the top cover/absorber Figure 2 Schematic of solar flat plate fluid heating device

Figure 3 Schematic of solar flat plate fluid heating device with fluid preheating facility

Figure 4 Schematic of solar flat plate fluid heating device with a facility of heating two different fluids simultaneously

Figure 5 Double side exposure solar flat plate fluid heating device

Figure 1 indicates three views of constructional features of the top cover and absorber. This sheet comprises of two surfaces 1 and 2 at the top and bottom separated by plurality of partitions (such as 4).

The partitions run parallel to each other along the length of the surfaces to form integrated conduits such as 3. Thus, surfaces'!, 2 along with the plurality of integrated conduits such as 3 separated by plurality of partitions forms a multi wall-sheet.

In one of the embodiments the multi wall-sheet comprises of more than two surfaces separated by plurality of partitions.

In another embodiments cavities/conduits may be of any cross sections

In yet another embodiment at least one surface of the multi-wall sheet of the absorber is translucent In another embodiment flat plate fluid heating device is a single multi-wall sheet with plurality of surfaces comprising top cover, absorber and insulating back covers wherein at least one of the top surfaces is translucent.

In another embodiment the top and bottom surfaces 1, 2 may be transparent, translucent, opaque or a combination thereof.

In yet another embodiment thickness of the surfaces 1, 2 may be a fraction of a millimetre.

In yet another embodiment material for the top and bottom surfaces is selected from plastics, polymers, glass, metals, alloys etc.

In another embodiment flat plate fluid heating device comprises of a header as outlet header disposed in between multi-wall absorber sheets wherein fluid is drawn in from the open conduits inlets of the absorber conduits which are disposed at different elevations.

Figure 2 revel details of the solar flat plate fluid heating device. Top cover 14 is in the form of a multi-wall sheet as described in Figure 1 , with translucent surfaces. Absorber consists of plurality of black coloured multi-wall sheets arranged/attached in steps such that part of the sheet overlaps subsequent sheet in series to form the arrangement as shown in the Figure 2 wherein multi-wall sheet 12 overlaps sheet 21 and 22 overlaps 12. These sheets are supported on supports 18 and 23 and oriented such that the integrated conduits in the sheets are parallel to the longitudinal axis of the inlet header 11. Fluid to be heated passes through 11 to inlet passage 19. Total fluid passing through 19 distributes so that part of the fluid passes through 21, other part through 12 and remaining through 22. Such a stepped arrangement of the multi-wall sheets facilitates distribution of the fluid flow resulting in significant reduction in the pressure drop and pumping power. Furthermore, heat transfer efficiency is enhanced by virtue of the fluid flow through the small conduits (with a very mall hydraulic diameter) in the multi-wall sheets. The geometry and construction of the multi-wall top cover results in substantial reduction of the top losses. Heated fluid passes to outlet header 13 through passage 20. Insulation 17, 16 fixed on a support 15 is provided at the bottom of the device.

In another embodiment inlet/outlet headers are selected from materials such as plastic, aluminium, FRP etc.

One of the embodiments of the solar flat plate heating device with a fluid preheating facility is shown in Figure 3. Fluid passes through inlet header 301 to the conduits in the translucent multi-wall sheet 304 placed on either side of 301. Fluid is preheated absorbing part of the solar energy incident on 304. Preheated fluid passes through the terminal passages 303 to the conduits of the absorber multi-wall sheet sections 309 and 310 further, heated fluid exits through the outlet passage 308. Insulation 307 and 306 is provided on a support 305.

Another embodiment of the solar flat plate heating device used for heating two or more fluids simultaneously is shown in Figure 4. First fluid to be heated passes through passage 410 to inlet header

408. Further, fluid passes to the conduits of the translucent multi-wall sheet 404, and gets heated absorbing part of the solar energy incident. The heated fluid passes to a passage 411 through outlet conduit 409. Second fluid to be heated passes through the passage 412 and inlet conduit 401 to the conduits of the absorber multi-wall sheet 402 and gets heated absorbing part of the solar energy incident. This heated fluid passes to a passage 414 through outlet conduit 403.

Figure 5 shows yet another embodiment with a facility of double side exposure of the absorber. Cross section of the device is shown in the figure. Plurality of stepped multi-wall absorber sheets 507 as described in Figure 2 are mounted on the plurality of the profiled glazed surfaces 508 provided with insulation 506. Fluid to be heated passes through the conduits such as 502. Part of the incident solar radiation passes through multi-wall top cover 509 and reflects from the surface of 508 on the bottom surface of the multi-wall absorber 502. Part of the solar radiation passing through 508 is absorbed on the top surface of 502. Thus absorber 502 has a double side exposure to the solar radiation. Top cover, absorber, glazed profile are enclosed with provisions 501, 503 and 505.

In another embodiment profile of the glazed surface is triangular, semicircular etc.

In yet another embodiment enclosure means is of materials such as plastic, FRP, Aluminium, metals etc.

With a judicious combination of translucent multi-wall sheet as a top cover with stepped multi-wall sheet absorber results in reducing top losses and pressure drop significantly thereby increasing efficiency while reducing weight of the said device.

Example

Experiments were conducted with the following objectives:

• To measure efficiency of the novel flat plate fluid heating device with air as a fluid being heated

• Compare the efficiency and weight of the novel heating device with that of the conventional flat plate device for the same aperture area, inlet air condition and insolation. Conventional device selected for the comparison is a single glass glazing flat plate collector with perforated absorber plate with selective coating. Results obtained for the conventional and the novel heating device of the present invention are as follows:

Particulars Present Invention Conventional Device

Inlet Air Temperature 29⁰C 29⁰C

Outlet Air Temperature 104⁰C 8O⁰C

Aperture Area 2 m² 2 m² H Heeaatt C Coolllleecctteedd 1 1117766 W W 800 W

Volume Flow Rate 780 lpm 780 lpm

Insolation 910 W/m² 910 W/m²

Efficiency 64 % 44 %

Collector Weight 12 kg 42 kg P Prreessssuurree D Drroopp 5 500 PPaa 250 Pa

It is evident from this invention that a judicious combination of translucent multi-wall sheet as a top cover with stepped multi-wall sheet absorber results in reducing top losses and pressure drop significantly thereby increasing efficiency while reducing weight of the said device.

Claims

What is claimed is:

1. A solar flat plate fluid heating device comprising: • translucent multi-wall top cover with plurality of integrated conduits;

• multi-wall absorber with plurality of integrated conduits;

• optional inlet or outlet fluid header/s for the absorber;

• optional inlet and outlet header/s for the top cover;

• insulation provision; • optional reflecting means placed under the said absorber; wherein translucent multi-wall top cover comprising top and bottom surfaces separated by plurality of partitions to form integrated conduits is positioned to cover the multi-wall absorber.

2. A solar flat plate fluid heating device as claimed in claim 1 wherein fluid is preheated by passing it through the conduits in the top cover before entering the absorber.

3. A solar flat plate fluid heating device as claimed in claim 1-2 wherein absorber consists of plurality of multi-wall sheets arranged to increase cross section area for fluid flow.

4. A solar flat plate fluid heating device as claimed in claim 1 comprising of a header as outlet header disposed in between multi-wall absorber sheets wherein fluid is drawn in from the open conduits inlets of the absorber conduits which are disposed at different elevations.

5. A solar flat plate fluid heating device as claimed in claim 1-4 wherein two or more fluids are passed through the top cover and absorber.

6. A solar flat plate fluid heating device as claimed in claims 1-5 wherein conduits in the top cover and the absorber are of the cross sections such as square, rectangular, oval, flat oval, circular etc.

7. A solar flat plate fluid heating device as claimed in claims 1-6 wherein material for the multi-wall top cover is selected from plastics, polymers, glass.

8. A solar flat plate fluid heating device as claimed in claims 1-7 wherein material for absorber is selected from plastics, polymers, glass, metals and alloys.

9. A solar flat plate fluid heating device as claimed in claims 1-8 wherein thickness of the surfaces of the multi-wall top cover and absorber may be as low as fraction of a millimetre but not limited to it.

10. A solar flat plate fluid heating device as claimed in claims 1-9 wherein flat plate fluid heating device is a single multi-wall sheet with plurality of surfaces comprising top cover, absorber and insulating back covers wherein at least one of the top surfaces is translucent.