Hybrid solar energy generating system

A hybrid solar energy generating system for effectively utilizing the total solar spectrum gathered by a solar ray concentrator. The system includes a first solar ray receiver having fluid-cooled photovoltaic solar cells for generating electricity and low-temperature heat. A second solar ray receiver is provided for generating high-temperature heat. A lens focuses the solar rays on the first receiver. A selective transmitting heat-mirror is positioned between the lens and the first solar ray receiver for reflecting selected portions of the solar ray spectrum to the second solar ray receiver and passing essentially the remaining solar ray spectrum to the first solar ray receiver. The heat-mirror reflects all solar rays having wavelengths longer than the long-wave spectral response cut-off of the photovoltaic solar cells and a selected part of the solar rays having wavelengths shorter than the long-wave spectral response cut-off. The heat-mirror spectral profile shape is modified to maximize the conversion of solar energy to high-temperature heat while causing only a minimal decrease in the generation of photovoltaic electricity.

BACKGROUND AND SUMMARY OF THE INVENTION 
This invention is directed to a hybrid solar conversion system which 
converts the total solar spectrum directly to energy and provides an 
energy output consisting of high-temperature heat, photovoltaic 
electricity, and low-temperature heat. 
An object of this invention is a hybrid solar conversion system that 
maximizes the high-temperature component of the energy output while 
maintaining the photovoltaic component at as high a level as is possible. 
Another object of this invention is a hybrid solar conversion system that 
improves the output efficiency by diverting energy from the photovoltaic 
cells that cannot be utilized by the photovoltaic cells to generate 
electric current and reflecting it to a high-temperature evacuated tube 
receiver. 
Another object of this invention is a hybrid solar conversion system that 
protects the photovoltaic cells against thermal degradation by diverting 
energy that cannot be utilized by the photovoltaic cells to generate 
electric current to a high-temperature evacuated tube receiver. 
Another object of this invention is a hybrid solar conversion system in 
which excess energy is diverted from the photovoltaic cells by modifying 
the heat-mirror spectral profile shape to maximize the conversion of solar 
energy to high-temperature heat while causing only a minimal decrease in 
the generation of photovoltaic electricity. 
Another object of this invention is a hybrid solar conversion system in 
which the heat-mirror spectral profile shape is modified to match the 
spectral response shape parameter of the photovoltaic cells so that the 
remaining incident spectral energy which cannot be utilized by the 
photovoltaic cells to generate electricity is converted to 
high-temperature heat. 
Another object of this invention is a hybrid solar conversion system which 
maintains the energy conversion efficiencies of each of its components 
essentially constant over the entire range of air mass which can be 
encountered during the calendar year. 
Other objects may be found in the following specification, claims, and 
drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 of the drawings is a schematic cross-sectional representation of the 
hybrid solar conversion system of this invention. The solar rays are 
depicted by arrows 1 passing through a linear Fresnel lens concentrator 3. 
In place of the Fresnel lens, a parabolic trough mirror may also be used. 
Although not shown for conciseness of disclosure, such a Fresnel lens 
concentrator would have a geometrical concentration ratio about 40.times. 
and an N-polar 2-axis tracking mechanism. The concentrated solar rays 
indicated by the numeral 5 are concentrated along an optic axis 6 to a 
virtual image at 7 through a cylindrical plano-concave lens 9. The lens 9 
may be made of Schott SF6 glass with anti-reflective coatings. This 
optical configuration has a high solar transmission factor of 0.96 with no 
shading factor. 
The exit rays 11 from the lens 9 reconverge on an image plane on the 
surface of solar cell 13 which is mounted in the solar ray receiver 15. 
The solar cell 13 consists of a linear array of high current solar cells. 
Solar cells particularly suitable for this application are supplied by 
Applied Solar Energy Corp. of City of Industry, California, under the 
designation x-Si solar cells which are single crystal silicon cells. It 
should be understood that other types of solar cells may also be used and 
the invention is not limited to any particular solar cell. In peak 
operation this array is designed to deliver 18 volts direct current. In 
normal operation, each module of the solar cells delivers 12 volts D.C. 
The solar cells are thermally anchored with an alumina loaded RTV 615A 
silicone to a copper substrate with a cooling channel 17. The solar cells 
13 and cooling channel 17 are embedded in a polyisocyanurate 
foamed-in-tube insulation 19. Reflective wing secondary concentrators 21 
are provided at the face of the solar cells 13 for improved image 
resolution. 
A selected transmitting heat-mirror 25 is positioned between the 
plano-concave lens 9 and the solar ray receiver 15 inclined at an angle of 
45.degree. to the optic axis of the rays between the lens and the receiver 
15. The mirror 25 spectrally splits some of the exit rays 11 from the lens 
9 and refocuses these rays 27 at an image plane at the aperture 29 of an 
evacuated tube receiver 31. The location of the evacuated tube receiver 31 
places it out of the path of incident rays to the solar ray receiver 15. 
The inner surface of the evacuated tube is coated with silver 33. A 
black-chrome coated absorber tube 35 is located in the evacuated tube to 
absorb the rays 27. A high-temperature silicone oil is pumped through the 
tube 35 to transfer the heat. Reflective wing secondary concentrators 37 
are provided at the aperture 29 for improved image resolution. 
The system as just described produces three types of energy from the total 
solar spectrum, namely, direct current electricity from the solar cells 
13, low-temperature heat from the water passing through the cooling 
channel 17 and high-temperature heat from the silicone oil passing through 
the absorber tube 35. The direct current electricity will be at 12 volts. 
The low-temperature water will be at approximately 50.degree.-70.degree. 
C. and the high-temperature silicone oil will be at approximately 
150.degree.-250.degree. C. The high-temperature heat and photovoltaic 
electric current are more useful than the low temperature heat. Therefore, 
the object of this solar energy system is to maximize the high-temperature 
heat and photovoltaic energy generated while minimizing the conversion to 
low-temperature heat. 
The selective transmitting heat-mirror 25 is used to divert the long 
wavelength solar energy and a selected portion of the short wavelength 
solar energy to the high-temperature evacuated tube receiver 31 in order 
to maximize the high-temperature heat while not significantly reducing the 
photovoltaic energy generated. Single crystal silicon solar cells such as 
x-Si cells have a quantum efficiency curve that utilizes only a portion of 
the short wavelength spectrum which passes through the heat-mirror. A 
typical Quantum Efficiency curve (QE) for an x-Si solar cell is shown in 
FIG. 2 of the drawings. Other types of solar cells will have different 
(QE) curves. In one aspect of this invention, a heat-mirror is selected to 
reflect not only all of the solar energy above a designated wavelength, 
but also a portion of the solar energy below a designated wavelength which 
cannot be utilized efficiently by the solar cells to generate electricity. 
This designated wavelength is called the long-wavelength spectral response 
cut-off. The long-wavelength spectral response cut-off for an x-Si solar 
cell is 1.1 .mu.m. Other types of solar cells have different spectral 
response cut-off wavelengths. A portion of the solar spectrum below 1.1 
.mu.m is customarily only partially converted to photovoltaic electricity 
because the Quantum Efficiency times Solar Flux (QE.multidot.S) profile of 
this solar cell is smaller than the original Solar Flux spectrum itself in 
this wavelength region. In my invention, the portion of the remaining 
incident spectral energy below the longwavelength spectral response 
cut-off resulting from over energetic photons and lattice thermal 
processes which is not utilized by the solar cell is converted to 
high-temperature heat. This is accomplished by use of special heat-mirror 
films which are applied to a glass substrate. The spectral profile of the 
heat-mirror is modified to match as closely as possible the Quantum 
Efficiency times Solar Flux (QE.multidot.S) profile of the solar cell as 
shown in FIG. 3. FIG. 2 shows a theoretical composite heat-mirror profile, 
where the transmittance peak is defined by the n=2 profile shown in FIG. 3 
and the infra-red portion above 1.1 .mu.m is set constant to equal 0.1. 
Theoretical transmittance profiles for heat-mirrors suitable for practicing 
my invention are shown superimposed on the Quantum Efficiency times Solar 
Flux (QE.multidot.S) profile of a single crystal silicon cell in FIG. 3 of 
the drawings. The theoretical transmittance profiles are represented 
parametrically by the formula T(HM)(x)=P[1-.vertline.x.vertline..sup.n ], 
where x is a wavelength parameter running from 0.fwdarw..+-.1, n is a 
shape parameter and P is a normalization factor. 
I have found several films which provide transmittance profiles which 
approximate the theoretical profiles shown in FIG. 3 of the drawings and 
thus closely match the QE.multidot.S profile of the single crystal silicon 
solar cell shown thereon. However, it should be understood that other 
films may also be useful with this and other types of solar cells. 
One type of film found suitable for this purpose is known as a tin doped 
In.sub.2 O.sub.3 (ITO) in which the transmittance/reflectance shoulder can 
readily be shifted with the level of the dopant. An example of an 
appropriate doping level to provide the proper transmittance/reflectance 
shoulder for this application is In.sub.2 O.sub.3 :8% Sn as reported by H. 
Kostlin, R. Jost and W. Lems in Phys. Stat. Sol., 29a, 87 (1975). 
Another suitable film for the heat-mirror of this invention is known as the 
interference dielectric-metaldielectric multilayer and is described in the 
article by J. C. C. Fan and F. J. Bachner, Applied Optics, 15, 1012, 
(1976). A heat-mirror with an appropriate spectral profile is shown in 
FIG. 4 of this article. This heat-mirror has a substrate of CG7059 glass 
which is 1 mm thick. Deposited thereon is a first layer of titanium 
dioxide, a second layer of silver and a third layer of titanium dioxide, 
each layer having a thickness of 180 angstroms. Other suitable heatmirrors 
of the ITO and interference dieletric-Au-dielectric (DAD) types may also 
be used as long as the shoulder is shiftable to a cutoff of 1.1 .mu.m 
wavelength. The spectral profile shape of the heat-mirror must be adjusted 
to maximize the conversion of the low-wavelength spectrum to 
high-temperature heat while permitting only a minimal decrease in the 
amount of energy converted to photovoltaic electricity.