A long-life solar reflector includes a solar collector substrate and a base layer bonded to a solar collector substrate. The first layer includes a first reflective layer and a first acrylic or transparent polymer layer covering the first reflective layer to prevent exposure of the first reflective layer. The reflector also includes at least one upper layer removably bonded to the first acrylic or transparent polymer layer of the base layer. The upper layer includes a second reflective layer and a second acrylic or transparent polymer layer covering the second reflective layer to prevent exposure of the second reflective layer. The upper layer may be removed from the base reflective layer to expose the base layer, thereby lengthening the useful life of the solar reflector. A method of manufacturing a solar reflector includes the steps of bonding a base layer to a solar collector substrate, wherein the base reflective layer includes a first reflective layer and a first transparent polymer or acrylic layer covering the first reflective layer; and removably bonding a first upper layer to the first transparent polymer or acrylic layer of the base layer. The first upper layer includes a second reflective layer and a second transparent polymer or acrylic layer covering the second reflective layer to prevent exposure of the second reflective layer.

FIELD OF THE INVENTION 
The present invention relates to a long-life self-renewing solar reflector 
stack formed by stacking a number of polymer reflecting films. The 
reflecting films are stacked and removably bonded together, and each layer 
is bonded less strongly than the previous layer. After a period of time, 
the top reflector layer may be peeled off to expose a new reflector layer 
underneath. 
The present invention also relates to a method of manufacturing a solar 
reflector by stacking multiple reflective layers which are bonded together 
by bonds of decreasing strength, thereby ensuring that each reflective 
layer can be easily peeled off to expose a new reflective layer. 
BACKGROUND OF THE INVENTION 
The process of channeling solar radiation to produce electric power is 
often accomplished through the use of heliostats, instruments consisting 
of one or more mirrors mounted on an axis which are moved by computerized 
clockwork to steadily reflect solar radiation in a predetermined direction 
(heliostat=sun's reflection in a constant direction). An example of a 
heliostat with two mirrors is shown in FIG. 1. 
The mirrors on heliostats or dishes which are used to focus the solar 
radiation, known as "reflectors," may be many meters in diameter and are 
constantly exposed to the environment. As a result, over a period of time, 
the surface of reflectors is degraded by this exposure, and the reflector 
is no longer useful. At this time, the surface of the reflector or the 
reflector itself must be replaced. 
Known methods for protecting the reflector surface from the environment 
include the use of laminated glass reflectors and plastic reflectors. 
Laminated glass reflectors include a reflective metal layer sandwiched 
between two glass sheets. These reflectors have a life span of 
approximately thirty years. They are scratch-resistant and therefore may 
be cleaned using ordinary soap and water. The glass layers are also 
impermeable to water and prevent water corrosion of the underlying 
reflective metal layer. However, glass reflectors are heavy and costly due 
to the increase in structural weight of the reflector, which is 
significant even when very thin glass is used. Glass is also vulnerable to 
hail and other impacts which result in exposure of the reflective metal 
layer. Glass is high in stiffness, and has a smaller coefficient of 
thermal expansion in comparison to the metal frame that supports the 
glass. Therefore, the glass must be applied as tiles of less than one 
square meter each. 
Plastic (polymer film) reflectors include a layer of transparent plastic 
covering a reflective metal layer. Plastic reflectors are lighter, less 
expensive, and more flexible than glass reflectors. Layers of plastic can 
be adhered to metal substrates as a single sheet as wide as 1.5 meters and 
10's of meters in length. Plastics have a higher coefficient of thermal 
expansion than metals, but their flexibility allows them to be bonded over 
large areas to metal support structures as a single piece. However, the 
plastic may be scratched during ordinary cleaning and therefore becomes 
dull over time. Furthermore, plastics are permeable and allow water to 
reach the underlying reflective metal layer, causing corrosion. As a 
result of these drawbacks, plastic reflectors have a life span of 
approximately seven to ten years. 
A possible solution to the degradation problem of plastic reflectors is to 
remove the worn outer plastic layer and replace it with a new layer to 
lengthen the usable life of the reflector. However, the process of 
replacing the outer plastic layer is cumbersome, results in a coating that 
is inferior to the original coating, and is expensive to the point of 
being economically infeasible. For example, the replacement process often 
results in particles from the environment being trapped between the 
plastic sheet and the metal structure layer, thereby distorting the 
reflector. Also, the replacement sheet may not be properly tensioned and 
sealed to the reflective layer such that moisture may seep in and corrode 
the reflective metal layer. 
SUMMARY OF THE INVENTION 
A long-life solar reflector according to the present invention includes a 
solar collector substrate and a base layer bonded to said solar collector 
substrate. The base layer includes a first reflective layer and a first 
transparent layer, such as an acrylic or transparent polymer layer, 
covering the first reflective layer to prevent exposure of the first 
reflective layer. The reflector also includes at least one upper layer 
removably bonded to the first acrylic or transparent polymer layer of the 
base layer. The upper layer includes a second reflective layer and a 
second transparent layer, such as an acrylic or transparent polymer layer, 
covering the second reflective layer to prevent exposure of the second 
reflective layer. The upper layer may be removed from the base reflective 
layer to expose the base layer, thereby lengthening the useful life of the 
solar reflector. 
Another embodiment of the long-life solar reflector according to the 
present invention includes all of the features described above and has two 
upper layers. A first of the two upper layers is removably bonded to the 
first transparent layer of the base layer, and a second of the two upper 
layers is removably bonded to the second transparent layer of the first 
upper layer. Furthermore, the second upper layer is less strongly bonded 
to the first upper layer than the first upper layer is bonded to the base 
layer. 
A method of manufacturing a solar reflector according to the present 
invention includes the steps of bonding a base layer to a solar collector 
substrate, wherein the base reflective layer includes a first reflective 
layer and a first transparent layer covering the first reflective layer; 
and removably bonding a first upper layer to the first transparent layer 
of the base layer. The first upper layer includes a second reflective 
layer and a second transparent layer covering the second reflective layer 
to prevent exposure of the second reflective layer. 
The apparatus and method according to the present invention as described 
above may provide the advantages of plastic reflectors and the life span 
of glass reflectors, i.e., a low cost, light-weight, easily applied 
reflector layer capable of a 90% solar reflectance average over thirty 
years. The apparatus and method according to the present invention may 
also have the additional advantage that all necessary reflective layers 
are applied to the reflector in the factory under ideal conditions to 
insure that each of the underlying reflective layers will be properly 
tensioned, flat, and properly sealed to prevent ingress of moisture. 
The foregoing and other features, aspects, and advantages of the present 
invention will become more apparent from the following detailed 
description when read in conjunction with the accompanying drawings.

DETAILED DESCRIPTION 
The reflector stack according to the present invention will now be 
described with reference to FIG. 2. The reflective stack illustrated in 
FIG. 2 includes three reflective layers. However, the present invention 
includes the use of two or more reflective layers in forming the reflector 
stack and is not limited to the three-layer structure shown in FIG. 2. 
As shown in FIG. 2, the reflector stack 200 includes three reflective 
layers 230, 231, and 232 which are applied to a solar collector substrate 
201. The top layer 230 includes a weathered acrylic layer 210 which has 
been exposed to radiation 221 from the sun 220 and other weather and 
environmental conditions; a silver reflector layer 209; and a low tack 
adhesive layer 208. The low-tack adhesive layer 208 bonds the top layer 
230 to the second layer 231 while allowing for easy removal of the top 
layer 230 from the second layer 231 by peeling the top layer 230 away. 
Thus, the top layer 230 may be peeled off to reveal a new reflective layer 
231 once the lifetime of the top layer 230 has expired. 
Similarly, the second layer 231 has an acrylic layer 207, a silver 
reflective layer 206, and a low tack adhesive layer 205. Low-tack adhesive 
layer 205 bonds second reflective layer 231 to a third reflective layer 
232, while allowing for easy removal of the second layer 231 by peeling 
the second layer 231 off to reveal the third reflective layer 232 once the 
lifetime of second reflective layer 231 has expired. 
The third reflective layer 232 includes an acrylic layer 204 and a silver 
reflective layer 203 like the layers 230 and 231 above it. However, third 
layer of 232 is bonded to the solar collector substrate 201 using an 
adhesive layer 202 which does not necessarily allow for peeling of the 
third layer 232 from the surface of the solar collector substrate 201. 
The three layers 230, 231, and 232 are bonded by adhesive bonds of 
increasing strength to insure that the top layer 230 may be removed 
without removing the second layer 231 or third layer 232, and that the 
second layer 231 may be removed without removing third layer 232. To 
achieve this result, third layer 232 is bonded to the solar collector 
substrate using an adhesive layer 202. The adhesive used in layer 202 
consists of, for example, approximately 30% glass microspheres (also known 
as micro-balloons). As a result, the adhesive in adhesive layer 202 
contacts approximately 70% of the surface of the solar collector substrate 
201. A full adhesive containing no glass microspheres may also be used to 
form adhesive layer 202. The bond formed between adhesive layer 202 and 
solar collector substrate 201 is the strongest of the bonds used in 
reflector 200. 
Next, the second layer 231 is bonded to the third layer 232 using a 
low-tack adhesive layer 205. The low-tack adhesive used in adhesive layer 
205 consists of, for example, approximately 50% glass microspheres. As a 
result, the adhesive in adhesive layer 205 only contacts approximately 50% 
of the acrylic layer 204 of the third layer 232. This creates a weaker 
bond between the second layer 231 and the third layer 232 than exists 
between third layer 232 and solar collector substrate 201. 
Then, the top layer 230 is bonded to the second layer 231 using a low-tack 
adhesive layer 208. The low-tack adhesive used in adhesive layer 208 
consists of, for example, approximately 80% glass microspheres. As a 
result, the adhesive in adhesive layer 208 only contacts approximately 20% 
of the acrylic layer 207 of the second layer 231. This creates a weaker 
bond between the top layer 230 and the second layer 231 than exists 
between second layer 231 and the third layer 232. 
The adhesive layers 202, 205, and 208 for use in the reflector stack 
according to the present invention must have sufficient tack to hold the 
reflective layers together to maintain an optically accurate surface and 
prevent the ingress of moisture between the reflective layers. The 
adhesive in adhesive layers 205 and 208 must also allow for the clean, 
complete removal of reflective layers 231 and 230 respectively at the end 
of their lifetimes without disturbing the underlying reflective layers. 
One such adhesive is a limited-tack or "low-tack" adhesive made by the 3M 
Company of St. Paul, Minn. in which the percentage of glass microspheres 
may be varied to produce bonds of varying strength. 
One possible type of film that may be used to construct reflective layers 
203, 206, and 209 and acrylic layers 204, 207, and 210 of the reflector 
stack according to the present invention is a highly specular 
silver-polymer (PMMA) solar reflecting film, 3M Brand ECP 305 Plus, made 
by the 3M Company of St. Paul, Minn. This film includes an acrylic (PMMA) 
layer, a silver layer, and a copper layer. Therefore, the reflective 
layers 203, 206, and 209 shown in FIG. 2 include a silver layer and a 
copper layer if 3M Brand ECP 305 Plus is used. However, other types of 
reflective films may be used without departing from the scope of the 
present invention. Also, other types of transparent films, for example, 
transparent polymer films, may be used in place of the acrylic films 
described above with reference to FIG. 2. 
The solar collector substrate 201 may be a metal foil/sheet or a mylar 
(polyester) substrate. However, other types of solar collector substrate 
materials may be used without departing from the scope of the present 
invention. 
While the present invention has been particularly described with reference 
to the preferred embodiments, it should be readily apparent to those of 
ordinary skill in the art that changes and modifications in form and 
details may be made without departing from the spirit and scope of the 
invention. It is intended that the appended claims include such changes 
and modifications.