Method for gaining heat energy from a solar collector and heat energy absorbent substances usable in this method

Abstract

The invention relates to a method for gaining heat energy from a solar collector, wherein energy is produced through a heat exchanger upon circulating a heat energy absorbent substance in a closed system of the solar collector. According to the invention fluid sulphur is used as heat energy absorbent substance in admixture with 0.5-15% by weight of selenium and/or tellurium, based on the total weight of the heat energy absorbent substance. The invention also relates to heat energy absorbent substances usage in the above method.

Description

The invention relates to a method for gaining heat energy from a solar collector. The invention also relates to heat energy absorbent substances usable for this purpose.

As known, under optimum position and weather conditions, an energy of about 1400 W/cm² is emitted onto the earth by solar radiation. It has been a long-standing need to utilize this energy. Solar collectors serve this purpose. The term “solar collector”, as used in this description and claims, encompasses all systems and equipment applied to gather energy of solar radiation.

The oldest solar collectors are simple black tanks or tubes wherein water, serving as heat energy absorbent substance, is stored or circulated under exposing it to solar radiation. The stored or circulated water absorbs about 5-6% of the heat energy of solar radiation, which is sufficient to warm it to 50-60° C. This method, which is the least effective utilization of solar energy, enables one to produce bath water under saving energy from other sources.

According to the up-to-date solutions solar radiation is directed in a concent-rated way by mirrors to the energy receiver of the solar collector, wherein a heat energy absorbent substance is circulated. The heat energy absorbent substance, which has been heated by solar radiation, is cooled then with water in a heat exchanger, and steam produced in the heat exchanger is utilized as energy source e.g. to operate a steam turbine for producing electric current. Such solutions are disclosed e.g. in U.S. Pat. Nos. 4,189,922, 4,335,578, 4,401,105, 4,488,540, 4,942,736 and 5,899,071. Mirrors and energy receivers used in such solar collectors may have the most diverse construction; as mirrors e.g. flat or parabolic mirrors or various combinations thereof, whereas as energy receivers tanks, tubes, pipelines, coils, etc. are applied.

A basic requirement for heat energy absorbent substance is that it should be a fluid which enables it to be circulated. Desirably, the specific heat of the fluid substance should be sufficiently high, because otherwise great amounts of fluid substance should be circulated in order to gain an acceptable amount of energy. Previously oil has been utilized as heat energy absorbent substance (U.S. Pat. No. 4,189,922), the use of liquid sulphur has also been suggested (U.S. Pat. No. 4,335,578), nowadays, however, the use of various molten salts is the most wide-spread.

As known, sulphur has a melting point of 120° C. and a boiling point of 446.5° C., which means that sulphur is liquid in the temperature range of 120-446.5° C., and is vaporous above the upper limit of this range. Liquid sulphur comprises sulphur in the form of S₈ molecules. Upon raising the temperature of liquid sulphur, S₈ molecules spontaneously start to convert into lower membered molecules. This conversion proceeds, however, very slowly, which means that sulphur enters the vapour phase also mainly in the form of S₈ molecules. The specific heat of fluid sulphur in the form of S₈ molecules is only about one-sixth of that of water. This explains why only a low efficiency can be attained by the method suggested in U.S. Pat. No. 4,335,578, and why the heat taken up by fluid sulphur can be used only to vapourize a low boiling organic liquid (toluene).

The most frequently applied types of molten salts are various mixtures of potassium nitrate, calcium nitrate and sodium nitrate, which are fed into the energy receiver as a melt of about 290° C., and are forwarded to energy production when of that of water, thus more than 1000 tons of a molten salt should be circulated in a solar collector in order to gain an appreciable amount of heat energy. This is, however, rather expensive, because the price of one ton of molten salt is 1500 USD in average. As a further disadvantage, molten salts are colourless or very light-coloured substances which are highly reflective, therefore they should be exposed to solar radiation always in a black or dark coloured receiver. Such black or dark coloured receivers transmit, however, infrared radiation only, therefore only the infrared portion of solar radiation is available as energy source for the heat energy absorbent substance. This explains primarily why solar collectors operated with molten salts make possible the utilization of only 18-20% of the energy radiated thereon. The use of molten salts has the further disadvantage that molten salts are liable to decomposition at about 500° C., which is the temperature of their utilization, and the decomposed part should be supplemented, which adds to operational costs.

The invention aims at providing a method and a heat energy absorbent substance which enables one to increase considerably the efficiency of gaining energy from a solar collector in a less expensive and environment-friend manner.

The invention is based on the recognition that fluid sulphur, in admixture with selenium and/or tellurium, is an excellent heat energy absorbent substance for use in a solar collector.

In order to avoid misunderstandings it should be stressed that the term “fluid sulphur”, as used in the description and claims, covers both liquid sulphur and sulphur vapours. Where only liquid sulphur or sulphur vapour is concerned, this is indicated separately.

It has been observed that the conversion of S₈ molecules to lower membered molecules (S₆, S₄ and finally S₂) is highly accelerated in the presence of selenium

It has been observed that the conversion of S₈ molecules to lower membered molecules (S₆, S₄ and finally S₂) is highly accelerated in the presence of selenium and/or tellurium additive. As a result of this phenomenon the specific heat of fluid sulphur raises to about the fourfold within an acceptably short time, thus it excellently absorbs the energy of solar radiation. This conversion proceeds partly even in liquid sulphur, but it is particularly intense in the vapour phase.

As a-further important advantage, liquid sulphur is of dark colour, which renders the use of black or other dark coloured energy receiver unnecessary. Thus liquid sulphur (and sulphur vapour formed therefrom) can also be exposed to solar radiation in a receiver made of a material which is transmittant within the 280-4000 nm wavelength range of solar radiation over a range exceeding the infrared one. Consequently, beside the infrared radiation transmitted by black or dark coloured materials, ranges of solar radiation between ultraviolet and infrared are also available to the heat energy absorbent substance as energy source. The energy receiver can be made of any suitable material which is transmittant over a range exceeding the infrared one. A particularly-preferred structural material for the energy receiver is quarz glass.

Based on the above, the invention relates to a method for gaining heat energy from a solar collector, wherein energy is produced through a heat exchanger upon circulating a heat energy absorbent substance in a closed system of the solar collector. According to the invention fluid sulphur is used as heat energy absorbent substance in admixture with 0.5-15% by weight of selenium and/or tellurium, based on the total weight of the heat energy absorbent substance.

Fluid sulphur may optionally also contain viscosity-lowering additives as discussed in U.S. Pat. No. 4,335,578. The term “fluid sulphur” as used herein also extends to fluid sulphur containing such additives.

According to a preferred method the heat energy absorbent substance is exposed to solar radiation in an energy receiver made of a material which is transmittant within the 280-4000 nm wavelength range of solar radiation over a range exceeding the infrared one. Preferably an energy receiver made of quarz glass is used.

Thus, in a further aspect, the invention relates to a heat energy absorbent substance for gaining heat energy from a solar collector, which comprises sulphur and 0.5-15% by weight of selenium and/or tellurium, calculated for the total weight of the heat energy absorbent substance.

Furthermore, the invention relates to the use of a material which is transmittant within the 280-4000 nm wavelength range of solar radiation over a range exceeding the infrared one, preferably of quarz glass, as structural material for an energy receiver of a solar collector.

The most important advantage of the invention is that it enables a substantial increase in the efficiency of gaining heat energy from a solar collector. Utilizing the method of the invention, the heat energy gained from a solar collector can be at least the double of that gained from a solar collector operated with a conventional molten salt. Further important advantages are as follows:

• the method of the invention is much less expensive than those wherein a molten salt is utilized;
• only minor structural changes should be performed on an existing solar collector to put the invention into practice;

EXAMPLE

The tube system exposed to solar radiaton, which serves as energy receiver in a laboratory model of a solar collector tower, was changed from the conventional black one to one made of quarz glass. A liquid heat energy absorbent substance, composed of 95% by weight of sulphur and 5% by weight of selenium, was fed at 133° C. into this tube system from a heatable storage tank by a circulation pump. In the tube system exposed to solar radiation vapours 450° C. in temperature were formed. The vapours were fed, at a rate of their development, into a buffer tank exposed to solar radiation, made of quarz glass, serving as a further energy receiver, where the temperature of the vapours raised to 480° C. The vapours with a heat uptake of 300-350 KCal/kg were cooled with water in a conventional heat exchanger to 170-180° C.; the steam developed in the heat exchanger was forwarded for energy utilization. The cooled liquid heat energy absorbent substance was recirculated into the tube system discussed above.

As long as sunlight can be reflected to the receiver by mirrors, the liquid heat energy absorbent substance remains in steady circulation.

Claims

1. A method for gaining heat energy from a solar collector, wherein energy is produced through a heat exchanger upon circulating a heat energy absorbent substance in a closed system of the solar collector, in which fluid sulphur is used as heat energy absorbent substance in admixture with 0.5-15% by weight of selenium and/or tellurium, based on the total weight of the heat energy absorbent substance.

2. A method as claimed in claim 1, in which the heat energy absorbent substance is exposed to solar radiation in a receiver made of a material which is transmittant within the 280-4000 nm wavelength range of solar radiation over a range exceeding the infrared one.

3. A method as claimed in claim 2, in which quarz glass is used as a material which is transmittant within the 280-4000 nm wavelength range of solar radiation over a range exceeding the infrared one.

5. Use of a material which is transmittant within the 280-4000 nm wavelength range of solar radiation over a range exceeding the infrared one as structural material for an energy receiver of a solar collector.

6. Use of quarz glass as structural material for an energy receiver of a solar collector.