GB2477001A

GB2477001A - Spectrally selective solar panel

Info

Publication number: GB2477001A
Application number: GB1010073A
Authority: GB
Inventors: Andrew Clifford Pate
Original assignee: Individual
Current assignee: Individual
Priority date: 2010-01-19
Filing date: 2010-06-16
Publication date: 2011-07-20
Also published as: GB201000787D0; GB2476981A; GB201010073D0

Abstract

A spectrally selective panel 7 comprises an essentially static permanent spectrally selective. liquid 1 which is mechanically bound to a solar transparent covering 2 in such a manner that the resulting structure does not significantly deflect, focus, scatter or absorb solar light. Preferably, the liquid is water, or a liquid containing hydrogen bonds, which absorbs heat radiation. The spectrally selective liquid may be bound to the covering by mixing it with a gelling agent or resin. Alternatively, the spectrally selective liquid (3, fig.4) may first be encapsulated within a sphere (4, fig.4) which is then bonded by resin or gel (5, fig.4) to the covering. Advantageously, the spectrally selective liquid may be quilted between two coverings (see figures 5 and 6). The spectrally selective panel may be combined with an absorber to form a solar collector, where the absorber preferably comprises an oil bed 9 flowing over a black surface 10 contained by a neoprene rubber membrane 11. Preferably, the spectrally selective panel is held in tension, or separated from other components of the solar collector, or otherwise made rigid, by the use of a pressurised gas, such that the solar collector may be inflated.

Description

Spectrally selective composite panel for thermal solar collection Backuround The principle of using light from the sun to heat a liquid or gas has been around many decades, and many technologies have been developed to convert this heat to electrical power, such as organic Rankine cycle (OCR) and sterling engines. However commercial solar power production still remains uneconomic due to the high cost of conventional thermal solar panels as well as the cost of more complex solar collection systems that track the sun and focus the sunlight onto a smaller absorbing surface.

To construct an efficient thermal solar collector (one that does not focus sunlight) requires a flat component that allows sunlight through, but prevents radiated heat from the collector's light absorbing surface passing back through and escaping into the environment; materials possessing this property are referred to as spectrally selective.

Sunlight has wavelengths ranging from 200 to 1500 nanometre reaching a maximum intensity at approximately 600 nanometre, whereas radiated heat from the collector's light absorbing surface has wavelengths in excess of 5000 nanometre reaching a maximum intensity at around 16000 nanometres.

According to the current state of the art several spectrally selective materials and coatings have been invented, for example V Kool 70 ® that allows 70 % of incoming Q sunlight to pass through while also reflecting around half of the heat that is radiating away from a collector's absorber back toward it. However these materials are costly to manufacture and apply (around £ 60 per square metre) and are usually based on thin metal films or interference filter technologies.

There exists a second type of spectrally selective material that sunlight can passes C) through easily, in which radiated heat gets absorbed internally rather than reflected from its surface. In the absence of other significant heat losses (such as convection or conduction) these materials warm until the heat re-radiated (in both directions) equals the heat that it is absorbing. This material then appears to be partially opaque to radiated heat because approximately half returns back to its source (Figure 1). The invention described herein is a spectrally selective panel of this type.

An example of this second type of spectrally selective material is glass which is usually used in several layers separated by an insulating gas which further enhances the blocking of radiated heat (Figure 2). Glass, Polycarbonate and Mylar ® all have the desired spectrally selective light absorption characteristic, but they need to be several millimetres thick in order to achieve this, which makes them relatively expensive and would result in collector costs in excess of £ 11 per square metre.

However there are several low cost solar coverings that are transparent to sunlight that are not very spectrally selective (for example polyethylene sheet that costs less than £ 0.50 per square metre) and also several low cost liquids that exhibit the desired spectrally selectivity, even when used in a thin layer. For example liquid water possesses multiple hydrogen bonds making it absorb heat radiation strongly over a wide range of wavelengths; because of this a thin layer of water a tenth a millimetre thick is sufficient to absorb almost all incident heat radiation.

Statement of invention

According to the present invention a layer of strongly spectrally selective liquid or gel, such as water, is combined with a low-cost covering transparent to sunlight to produce a low cost spectrally selective composite panel. The spectrally selective composite panel does not contain additional features claimed within prior art that make it a barrier of low thermal conductivity or to causing it to deflect, focus or control the scattering or absorption of sunlight. Omitting these additional features allows the cost of the spectrally selective composite panel to be reduced.

Preferably, the liquid is permanent and is not under significant flow as the liquid is not used to transport heat. This also allows the liquid to be attached to the covering by the use of sol-gel capsules, adhesives and gelling agents.

Preferably, the spectrally selective panel may be held in tension or separated from other components of a solar collector, or otherwise made rigid, by the use of a pressurised gas; in other words the solar collector assembly may be designed to be inflated. The resulting inflated solar collector does not form or include a rigid concave

reflector, as may be present in prior art.

Preferably, a solar collector containing a spectrally selective composite panel may also include a transparent covering that is not spectrally selective at the top in order to retain a layer of warm static air above the components below.

Preferably, a solar collector containing a spectrally selective composite panel may have reflectors situated around it in order to improve its performance.

0 This patent differs from prior art in which liquids are enclosed to form optical component such as a prism or lens, or are used to move heat. It is also not like sun- tracking solar collector designs that that employ a pressurised gas in order to form a rigid concave reflector.

Advantages The present invention should cost less to manufacture than existing spectrally selective panels comprised of materials such as Glass, Polycarbonate and Mylar ®.

A solar collector using the present invention needs no additional elements or structures to track the daily motion of the sun. This is because it does not focus sunlight onto an absorber that is smaller in area, as in patents (US2008/236569 A (TUCCIO) and (JP2008185731 A (PAUL & CHARLENE)).

The present invention is a low cost solution addressing the main source of energy loss in solar collectors, which is heat radiating away from the absorber into the environment. The present invention is therefore more applicable to commercial solar power production than prior art where less significant losses are addressed at some cost, such as the conduction of heat through the panel or reflection losses occurring within panel.

Patent (DE383 149 C (BOETTCHER)) creates a tow thermal conductivity barrier through which sunlight can pass by immersing an insulating layered, honeycombed or irregular particulate structure within a transparent liquid. Specifically this patent uses a liquid with a very similar refractive index to the materials around it (or within it) to reduce the scattering of sunlight. Since the amount of sunlight scattered at the boundaries between common materials such as water and plastic is only slight (around 0.5%) this feature is only of practical importance if the panel is designed to act as a low thermal conductivity barrier. If the panel is designed to act as a low thermal conductivity barrier it will contain within the liquid, solid structures to suppress convection, these structures create many more boundaries within the light-path. Since the present invention does not act as a low thermal conductivity barrier, liquids of any refractive index can be used thereby reducing the cost.

Patent (DE4037 136 C (DEGUSSA)) creates a low thermal conductivity barrier through which sunlight can pass by immersing thermally insulating pyrogenic silica tiles (Aerosile) within liquid paraffin. Although the patent indicates the resulting structure has spectrally selective properties it also has a complex structure different from the present invention. Moreover the paraffin layer is relatively thick so the resulting structure is thicker, heavier, more flammable and more expensive than the invention described herein.

In the prior art there also exists several inventions in which liquid flows solar Q transparent panel either to provide cooling or to distribute heat. This differs from the present invention wherein the liquid is essentially permanent and not under significant flow. For example Patent (U54368725 A (MCCLINTOCK)) uses the liquid under flow to distribute heat; this patent also employs a relatively complex prism structure Q that allows the sunlight to be deflected. Patent (FR247828 1 A (OPTHRA)) mentions the use of water under flow to provide cooling to the other panel components. Neither of these patents makes reference to the liquid used being spectrally selective.

Examples

Several preferred embodiments of the invention are described now by way of example with reference to the accompanying drawings:-Figure 3 shows in cross-section a spectrally selective composite panel where the spectrally selective fluid 1 is bound to the transparent covering 2 by mixing it with a gelling agent or resin.

Figure 4 shows in cross-section a spectrally selective composite panel where the spectrally selective fluid 3 is first bound within a Sol-gel sphere 4 (not shown to scale) which is then bonded by resin or gel 5 to the covering 2. The Sol-gel capsule may prevent the bonding S altering the characteristics of the spectrally selective fluid 3, or extend the lifetime of the panel.

Figure 5 shows in cross-section a spectrally selective panel where the spectrally selective fluid 3 is quilted between two coverings 2.

Figure 6 shows an illustration of a spectrally selective panel where the spectrally selective fluid has been quilted between two coverings.

Figure 7 shows in cross-section a solar collector design where pressurised gas 6 separates the spectrally selective panels 7 from each other and the absorber which may be comprised of an oil bed 9 flowing over a black surface 10 contained by a neoprene rubber membrane 11.

Figure 8 shows in cross-section a solar collector design as described in Figure 7 which possesses radial symmetry about a central axis 12. The resulting solar collector formed would be dome-like or geodesic in structure. Such a structure is likely to be significantly more rigid and wind-resistant than that described by Figure 7.

Figure 9 shows in cross-section a solar collector design as in Figure 7 which uses a vertically mounted reflector to increase the amount of light captured when the sun is low. (0 C) (\J

Claims

Claims 1 A spectrally selective panel that comprises of an essentially static permanent liquid intended to absorb and re-emit heat radiation that need not have a similar refractive index of the materials around it in order to reduce the scattering of sunlight, nor is used to distribute heat, nor acts as a low thermal conductivity barrier, that has been mechanically bound to a solar transparent covering in such a manner that the resulting structure does not significantly deflect, focus, scatter or absorb solar light.
2 A spectrally selective panel as claimed in any preceding claim where the spectrally selective liquid is water.
3 A spectrally selective panel as claimed in any preceding claim where the spectrally selective liquid contains one or more hydrogen bonds.
4 A spectrally selective panel as claimed in any preceding claim where the spectrally selective liquid contains additives to prevent freezing.
A spectrally selective panel as claimed in any preceding claim where the spectrally selective liquid contains additives to prevent microbes forming.
6 A spectrally selective panel as claimed in any preceding claim where the spectrally selective liquid contains a gelling agent.
7 A spectrally selective panel as claimed in any preceding claim where the (0 spectrally selective liquid is encapsulated within a Sol-gel.

Q
8 A spectrally selective panel as claimed in any preceding claim where the spectrally selective liquid or Sol-gel is mechanically bound to the solar transparent C\I covering by use of an adhesive, gel or resin.
9 A spectrally selective panel as claimed in any preceding claim where the spectrally selective liquid is mechanically bound to the solar transparent covering by use of a second solar transparent covering to form a layered or quilted structure.
A spectrally selective panel as claimed in any preceding claim where the solar transparent covering is glass or one of the following polymers Polyethylene, Acrylic, Polycarbonate, Mylar ® , FEP, PVF, PMMA, PETP or FRP.
11 A spectrally selective panel as claimed in any preceding claim where the solar transparent covering or spectrally selective liquid could be devised by someone reasonably skilled in the art.
12 A solar collector assembly that uses a spectrally selective panel as claimed in any preceding claim that could be devised by someone reasonably skilled in the art.
13 A solar collector assembly as claimed in any preceding claim wherein the top layer is a conventional transparent panel used to retain a layer of warm static air above the components below.
14 A solar collector assembly as claimed in any preceding claim wherein the spectrally selective panel is held in tension or separated from other components of the solar collector, or otherwise made rigid, by the use of a pressurised gas; in other words the solar collector assembly is designed to be inflated.
A solar collector assembly as claimed in any preceding claim where a reflector is used to increase the amount of light captured.
16 A solar collector assembly as claimed in any preceding claim where the pressurised gas, is a greenhouse' gas for example carbon dioxide. (0 C) (\J