The present invention relates to a solar energy collector. Collectors of solar energy in the form of heat are well known. Typically they take the form of tubes, which can be in the simply form of a continuous pipe, arranged to receive radiation from the sun and heat water passing through it. More

sophisticated tubes are in the form of heat pipes, arranged with an upper heat exchanger. Generally such devices are used for domestic purposes and there is little demand for water to be heated above domestic use purposes.

It is known to use liquids other than water in non-heat pipe collectors if only to avoid freezing. The object of the present invention is to provide an improved solar energy collector, in particular a solar energy collector able to efficiently capture solar energy.

According to a first aspect of the invention there is provided a solar energy collector comprising:

• an array of a plurality of transparent or at least translucent tubes;

• a pair of manifolds between which the tubes extend and with which they are internally in communication; and

• a heat transfer liquid to be heated on circulation through the tubes and

manifolds, the liquid including black absorbent material for solar heat absorption.

According to a second aspect of the invention there is provided use of a heat transfer liquid including black absorbent material in a solar collector having a plurality of transparent or at least translucent tubes extending between a pair of manifolds via which the liquid is passed to and from the tubes for solar heat absorption. According to a third aspect of the invention there is provided a heat transfer liquid for absorbing solar heat, the liquid including black absorbent material.

According to a fourth aspect of the invention there is provided a method of absorbing heat consisting in the step of:

• passing heat transfer liquid including black absorbent material through a solar collector having a plurality of transparent or at least translucent tubes extending between a pair of manifolds via which the liquid is passed to and from the tubes for solar heat absorption.

As a black absorbent material, we prefer to use carbon black.

Carbon black is an amorphous or paracrystaline form of microscopic particles of carbon in graphitic state. It is a substance which readily absorbs solar radiation, the heat energy bearing part of the spectrum of radiation from the sun.

The liquid is preferably one having a higher boiling point than water. We expect the following to be suitable, their boiling points being indicated organic compounds such as glycerine, 290°C, ethylene and propylene glycol, 197°C & 187°C and possibly synthetic oils such as silicone oils. We would prefer not to use ethylene glycol despite its apparent suitability, due to its toxicity.

We prefer to use between 0.75 weight percent to 1.25 weight % carbon black to bulk liquid. However we expect 0.5 weight % to 2.0 weight % to be suitable. This quantity of carbon black forms a colloidal suspension from which carbon black does not precipitate.

In the preferred embodiment, the transparent / translucent tubes are set within U-shaped reflectors arranged to reflect radiation incident between the tubes onto them. Normally, we will install the tubes and the reflectors in a plane substantially normal to the average elevation of the sun. We prefer to install the tubes and their reflectors across the plane as opposed to up and down it, to avoid shading of the tubes by the reflectors. Further the reflector is preferably provided with a median plane cusp with the tube abutting the cusp, to obviate radiation being able to pass under the tube.

According to a fifth aspect of the invention there is provided a solar energy collector comprising:

• an array of a plurality of transparent or at least translucent tubes;

• a corresponding plurality of U-shaped reflectors, a respective tube arranged within each reflector to receive solar radiation directly and by reflection; and

• a pair of manifolds between which the tubes extend and with which they are internally in communication;

wherein:

• each reflector has a cusp centrally of its U shape and

• the reflector's tube is placed against the cusp to avoid reflected solar radiation passing between the reflector and the tube and out again from the reflector without being incident on the tube.

Preferably the width of the reflector at the mouth of the U is between 2 and 5 times, and in particular between 2.S and 4.25 times, the diameter of the tube.

To help understanding of the invention, a specific embodiment thereof will now be described by way of example and with reference to the accompanying drawings, in which:

Figure 1 is a perspective view of a solar energy collector according to the invention;

Figure 2 is a cross-sectional side view of the collector;

Figure 3 is a more detailed view of the shape of a reflector and tube of the reflector and

Figure 4 is a block diagram of a thermal system in which the collector is incorporated.

Referring to the drawings, the solar energy collector 101 is generally planar and normally installed to face generally south (in the Northern Hemisphere) at an inclination to be generally perpendicular to the sun's average elevation. Insofar as the installation details are essentially conventional, they will not be described further, save to say that it comprises a casing 102 having a transparent window 103.

The collector has a plurality of east-west oriented (as installed) glass, preferably borosilicate glass, tubes 105, preferably 10mm in diameter, connected at their ends to transverse inlet and outlet manifolds 106,107. These are oriented up and down the inclined in used collector and allow fluid to be passed to the tubes for heating and withdrawn heated from them. The manifolds are mounted, suitably insulated, in the case and carry the glass tubes.

Behind the tubes each has a concave channel or generally U-shaped reflector 108 with a cusp 109 centrally in the bottom of its U. The reflector is mounted in the casing such that the cusp touches or is very close to the tube. The shape of the reflector is such that sunlight incident on the reflector is reflected onto the tube, without being able to pass under the tube. For our preferred tube diameter of a nominal 10mm, we prefer a reflector orifice width of 38mm. With these dimensions, the formula for the preferred shape of tube is:

This shape is semi-elliptical except for the flat sided angular cusp 9. The tube is placed at the focus of the reflector. In this embodiment, the reflector channel to tube diameter is 3.8:1.

This embodiment provides that, across the possible angles of incidence of solar radiation with seasonal variations, all incoming radiation is directed onto the tubes containing the 'black fluid'. The angle of the sun's rays varies with time of day (by up to ±90°) and time of year (by up to ±23.4°); orientating the tubes horizontally rather than vertically ensures that the greater of these two ranges, the daily one, has no effect beyond the inevitable cosine rule, while the annual variation is accommodated by the design of the reflector. Consistent with conventional practice, the panel as a whole is positioned slanted at an angle equal to the latitude of its location, oriented southerly in the northern hemisphere or northerly in the southern hemisphere, so that the panel is normal to the angle of the sun at local equinoctial noon and the seasonal variation of solar angle is symmetrically distributed about that position. The manifolds are connected to a circulation system 111 for heat transfer liquid to be heated by the solar energy collector. Typically this includes apparatus 112 in which heat energy is extracted from the liquid, typically a heat engine. Thence it is passed to a reservoir 114, which is open to the ambient atmosphere to maintain the liquid substantially at atmospheric pressure. For return to the collector, a pump 115 is provided. Loss of heat from the interior of the panel is minimised by means of a front glass 103 and insulation 131 separating the reflectors 108 from a back plate 132. Mechanical integrity for the panel as a whole is provided by an outer frame 102.

In accordance with the invention, the heat transfer liquid has carbon black mixed in with it, in a weight percentage of 1 %. The carbon black acts to absorb energy in incident solar radiation and transfer it to the heat transfer liquid, heating it.

The heat transfer liquid is an elevated boiling point liquid, i.e. higher than water's boiling point. Glycerine or a mixture of glycerine and propylene glycol are amongst the liquids that we prefer. We prefer to use glycerol (propane- 1 ,2,3 -triol) with an addition of propane- 1 ,2-diol.

A control system 121 for the pump is provided. It can control the circulation of the liquid to provide that the liquid leaving the collector is at a predetermined elevated temperature as measured by a thermometer 122 at an appreciable margin below the liquid's boiling temperature, i.e. typically at an elevated temperature of 150° C. Where the temperature is tending to rise, the liquid is pumped faster and vice versa. Where a dangerously high temperature is detected, as in low use of the heat by the apparatus 12, the pump can be reversed to pump the liquid from the collector to avoid its overheating. With the black liquid absent from the tubes, incident radiation mostly passes back out from them, avoiding their overheating. The black liquid is prepared by mixing an aqueous dispersion of carbon black with bulk liquid to achieve a 1.0 weight % carbon black to bulk liquid.

An advantage of the above described embodiment is that by concentrating solar radiation over the 38mm width of the reflectors onto 10mm wide glass tubes the liquid in the tubes can be heated to a temperature such as 150°C for use at that temperature. Glycerine is used since water would boil at this temperature (unless contained at unrealistically high pressure). In other words, the above described panel is capable of directly producing higher temperatures than existing solar-thermal panels.

Further the above described panel allows a heat-collecting fluid to be circulated simply between the panel and the point-of-use of the energy, giving highly efficient capture and transport of heat, and requiring no components under either vacuum or pressure.

As will be appreciated, the panel is based on the principle of 'black body absorption'. The invention is not intended to be restricted to the details of the above described embodiment. For instance, the manifolds can be differently arranged. At one end, half the manifold can be an inlet to half the tubes and the other half can be an outlet to half the tubes. At the other end, the manifold is common to all the tubes. This arrangement provides for the heat transfer liquid path in the collector to be twice the length of the tubes.