The present disclosure relates to the field of solar energy and to canopy and solar technology. Canopies have historically been utilized to provide shade for an area, and certain canopies have been fitted with various means of generating electrical power. For example, traditional forms of solar concentrators are based on flat or parabolic mirrors. Concentrated sunlight may be focused onto a receiving tower; thermal energy from the concentrated sunlight drives a turbine and generates electricity. In contrast, a typical parabolic trough CSP system uses a linear parabolic reflector to concentrate the sunlight onto a receiver positioned along the trough's focal line. The receiver is a tube (often called a Dewar tube) filled with fluid which, when heated, can similarly be used to drive a turbine and generate electricity. A tracking system may be used on the parabolic trough to ensure that it maximizes solar receipt during daylight as it tracks the sun along a single axis. Traditional technologies motorize the structure in order to ensure that the solar mirrors track the sun's path across the sky and concentrate the maximum amount of solar radiation on a central solar receiving tower or on a central pipe. In these conventional systems, a need to constantly adapt to the sun's moving path across the sky may be met through the employment of a highly mechanized, energy intensive motorized system.
Certain canopies are not as efficient as possible in terms of electrical power conversion and do not address other issues such as dust and dirt collection on reflecting surfaces, and/or liberating heat at night that concentrates in or under the canopy during the day.
In an embodiment, a solar canopy system includes first and second support towers and an upper canopy suspended at least from the first and second support towers, the upper canopy forming a catenary shape.
In an embodiment, a method of generating electricity includes deploying a plurality of mirrors upon a canopy that forms a catenary shape to focus sunlight on a Dewar tube containing a heat transfer fluid, utilizing heat from the sunlight collected within the heat transfer fluid to generate steam, and generating the electricity with a turbine powered by the steam.
In an embodiment, a method of generating electricity includes deploying first and second grids of photovoltaic cells upon a canopy that forms a catenary shape, the second grid being movable relative to the first grid, each of the first and second grids being operatively configured to generate electricity. The method also includes positioning the second grid with respect to the first grid so as to maximize light collection and electricity generation during daylight hours, and positioning the second grid with respect to the first grid so as to maximize transparency of the canopy during nighttime hours.
The present disclosure may be understood by reference to the following detailed description taken in conjunction with the drawings briefly described below. For purposes of illustrative clarity, certain elements in the drawings may not be drawn to scale, and only representative features of several similar features may be labeled. Specific instances of an item may be referred to by use of a numeral in parentheses (e.g., first tower 120(1)) while numerals without parentheses refer to any such item (e.g., towers 120).
Two solar canopy systems are presented herein, one based on CSP (concentrated solar power), and one based on PV (photovoltaic) cells.
1. CSP Solution
Solar canopy system 100(1) features an upper canopy 110 including flat mirrors 170 mounted with a special tensile geometry in such a way that mirrors 170 can be moved by simple movement of a cable (see
2. Technical Description of the CSP Solution
Solar canopy system 100(1) includes solar mirrors 170 coupled with upper canopy 110.
As shown in
Referring again to
3. PV Solution
A second embodiment of a solar canopy system employs a similar overall structure, but a difference between the first system and the second system is the way of collecting energy, now based on PVs. However, if one were to cover the whole surface of a tensile membrane with PVs, several issues may occur:
In the PV solution now described, the canopy includes multiple layers of PV cells in gridlike patterns that harvest solar energy, and can open and close when the layers make small movements relative to one another (see
4. Technical Description of the PV Solution
An embodiment is shown again based on a double catenary system. The system according to the inventive concept has a similar overall structure to the first system, with a dual catenary structure fixed together by tension cables and additional support from columns. Similar to the first system, the second system is semi-retractable/movable, can produce energy through solar harvesting and is capable of providing shade and ventilation.
The mesh like system of grid elements allows for a responsive surface, and is capable of producing varying degrees of shading. The PV cells that are integrated within the upper membrane of the catenary harvest solar energy and produce electricity on exposure to sunlight. In order to maximize solar receipt during daylight hours, the grids remain closed, that is, the grids are moved such that almost all available sunlight falls on the PV cells. This also serves to provide shade to the under-canopy area. During the night the grids open up, that is, the grids are moved such that the PV cells generally overlay one another and the spaces therebetween also open up. If one had a 4 layer set of grids one would be able to open it up to provide an open area of approximately 75%. While opening and closing, also, one cleans the PVs from dust—a major problem in hot desert climates. The lower membrane is an insulating layer, and creates a cavity between the two membranes. The solar energy that is not captured by the PV cells gathers as thermal energy between the membranes, and is restricted from permeating the canopy catenary structure by the insulation. In order to facilitate the removal of this layer of heat, the double membrane form allows for the creation of convection currents which dispel the hot air and also help to produce a slight breeze in the underlying region. The upwards flow of hot air draws in cooler air into the cavity, in turn creating a breeze in the under canopy area. The mobility of the grids also enables the canopy to be opened at night, allowing heat trapped below to dissipate and enhancing outgoing radiation.
5. CSP Solution with Conservation of Etendue
5.1. Introduction
In the public eye photovoltaic (PV) cells are the face of solar energy. This reflects the reality of solar energy production in the world—at date PV produces about 40 GW per year worldwide while its major alternative—concentrated solar power (CSP)—produces approximately 1.17 GW at a few plants, mostly in Spain and in the United States. PV is also relatively cheaper than CSP and has a great flexibility in how and where it may be installed. In fact, personal PV systems are becoming ever more popular and it has been shown that a rooftop solar PV system in Los Angeles is cheaper than even the most cost-effective CSP plant in the world—the PS10 solar power tower in Spain.
However, this disparity between the two technologies does not tell the whole story. In the race to improve solar collection technology, CSP has been left in a more rudimentary state, and its considerable potential has remained untapped. While today CSP exists only on large-scale power plants far from urban areas, we find that it may be uniquely suited for integration into urban architecture in hot climates. In addition to collecting solar energy, CSP offers the possibility of creating cool spaces underneath its cover; as opposed to PV cells—which transform a large fraction of incoming solar energy into heat—CSP uses mirrors which reflect incident light away. In the sun-soaked regions of the world where solar energy production is most feasible the creation of cool, sheltered spaces is one of the main goals of architecture, and CSP may be tailored to provide this function.
In an embodiment, a Solar Concentrating Roof (SCR) is provided utilizing one form of CSP-Compound Linear Fresnel (CLFR) technology. We consider how the solar collection efficiency of the roof may be maximized together with the capacity of the roof for climate control and other architectural concerns. By designing to reflect solar energy away while letting long-wave radiation through, the roof will create an optimum climate, following in the vein of the domes which Buckminster Fuller proposed could cover an area of Manhattan or the entire city of Winooski, Vt. (USA). Architecturally, the design will be light and inexpensive to build. Maximization of energy collection will be driven by the concept of the “conservation of etendue” of the incoming light. Our design optimizes functionality by considering maximum energy collection, climate control, and architecture together.
6. Energy Collection
6.1. Conservation of Etendue
A recent study by Chaves and Collares-Pereira shows how imposing the conservation of the optical quantity etendue on a CLFR reflector can lead to the development of new, potentially more efficient forms for the reflector. Etendue is an important concept in non-imaging optics. Essentially it is the space—area and angular area—which light occupies at a given point along its path. Etendue can increase but cannot decrease; this is based on the physical principle that concentrated light can be diffused while diffuse light cannot be reconcentrated. If etendue could be decreased then light coming from a finite source could be focused into an infinitely small area, infinitely high concentrations of energy could be attained, and the second law of thermodynamics would be violated. Therefore the etendue of light as it enters an optical system sets an upper limit for the concentration which can be achieved. CSP optics therefore work at their greatest efficiency when etendue is conserved. CSP solutions such as parabolic mirrors and converging lenses which do not conserve etendue always fall short of the maximum concentration. Etendue is defined (in two dimensions) as:
U=2nS cos γdθ (1)
where n is the index of refraction of the medium, S is the area (length in two dimensions) which the light strikes or passes through at an angle γ to the normal of the surface, and dθ is the half-angle aperture (solid angle) of the light.
Our problem is two-dimensional—two tower receivers with a field of heliostats between them. The heliostats are infinitely small and are alternatedly directed at the two receivers such that incident light at any point is split and directed to both receivers. The local slope is optimized such that etendue is perfectly conserved before and after incidence, that is:
dUI=dUL+dUR (2)
where dUI is the etendue of the incident light at a point P on the surface and dUL and dUR are the etendue of the light reflected from this point towards the tower receivers at the left and right-hand side of the heliostat field, respectively. The etendue of the incident radiation is:
dUI=2dl cos α sin θ (3)
and similarly:
dUL=2dl cos(φL−α)sin θ (4)
dUR=2dl cos(φR+α)sin θ (5)
Where φL and φR are the angle which the light reflected to the left and right receivers, respectively, make with the vertical. As such, φL and φR will be functions of the position of point P and the parameters of the system (span, height of receivers). Given that the solid angle of the radiation remains the same before and after reflection, the conservation of etendue dictates that the area (or in the 2D problem, the length) which the light passes through before reflection must be equal to the sum of the two areas which the light passes through after reflection. Consideration of Eq. (2) together with Eqs. (3), (4), and (5) leads to a differential equation:
which, given an initial value, defines the shape of an etendue-conserving curve. Chaves and Pereira find one solution of the differential equation; perhaps the solution which is optimal for a structure constructed on the ground. But, as we will show, other solutions may be of interest in the design of an elevated structure.
cos φL=cos φR=0.5 (7)
which can only be satisfied by one point, where we will have φL=φR=60° and, consequentially,
Outside of this point there are an infinite number of solutions.
We can go further and calculate the energy flux incident at the point P:
dΦI=LdUI (8)
where L is the radiance of the radiation. Etendue is conserved, and if we assume that there is no absorption, radiance is as well. Therefore flux will be conserved and the total flux collected H will be:
We conclude that for an etendue-conserving curve the amount of solar energy collected will not depend on the shape of the solution.
6.2. Variation of Sun Zenith Angle
We can also generalize the solution to accommodate radiation arriving from different sun zenith angles—the current solution considers only direct overhead radiation. θZ is 90° when the sun is in the east, 0° when the sun is over-head (noontime) and −90° when the sun is in the west. The etendue of the incident radiation is now:
dUI=2di cos(α+θZ)sin θ (12)
but the etendues of the reflected radiation have the same definition, as the direction of the reflected light remains unchanged:
dUL=2dl cos(φL−α)sin θ (13)
dUR=2dl cos(φR++)sin θ (14)
and the differential equation for the etendue-conserving curve becomes:
6.3. Corrections
The radiance of light is reduced by absorption as it travels through the earth's atmosphere. This reduction is referred to as air mass and has been approximated as:
where θz is expressed in degrees. We incorporate the air mass reduction into the radiance:
and we can therefore find the total energy collected by the two receivers at any time t:
Note that the correction generally known as the cosine effect is automatically included in the calculations through Eq. (12).
7. Climate Control
We noted earlier that the Concentrated Solar Canopy is an expensive option when compared with more conventional photo-voltaic approaches. But this may be a somewhat restricted perspective.
The CSL is a fundamentally more attractive option in that the heat is at high temperature as it is used to produce electricity; a high efficiency process. Of course the large array of carefully placed mirrors is expensive but this can be mitigated in part by finding a use for a large array of carefully placed mirrors, e.g., they should make an excellent reflector for incoming short wave radiation.
Turning to related architecture we note that the same pressures that have led to research and improvement of photovoltaics have also led to a reconsideration of building design with particular attention to the heating, cooling and ventilating of buildings. Concurrent with this has been a reinvigorated interest in space and the nature of space influencing current working and living patterns.
Central to these concerns is the general requirement of getting the right kind of energy at the right place and at the right time as a guide to sustainability. An inversion of this is to ensure that you don't have the wrong energy where you don't want it when you don't want it. In a hot or very hot climate and during the day the buildings will absorb and reflect short wave radiation from the sun thereby increasing the buildings temperature. Some of this will be re-radiated as long wave radiation and some transferred to the air surrounding the building. Both the direct human impact of incoming short wave radiation, the increased building temperature and the increased air temperature. all act to make the environmental temperatures uncomfortable in the extreme.
Experience shows that the incoming short wave radiation is particularly uncomfortable and we typically remain inside during much of the day, stay in shadow when outside or erect some, possibly light weight, structure to provide shade. To an extent though this still leaves the heated air and the heated building to contend with.
At night there is no incoming short wave radiation but the air and the building remain hot. The shade canopy is now somewhat of an inconvenience as it is likely to trap outgoing longwave radiation from the buildings and to trap the heated air even if it is hot and positively buoyant. Additionally the shade canopy reduces the possibility of wind flushing the hot air out of the urban canopy.
The obvious solution is to remove the shade canopy at night. Thus the CSL becomes a very attractive option if the shade canopy can provide shade and energy during the day and can be moved or altered (through limited movement) to provide substantial cooling and reduction in temperature of the urban canopy of buildings and the air within the urban canopy.
Finally, we can note that active Climate Control as described above will result in reduced energy consumption in the cooling of buildings and thereby likely impinge on concerns about Climate Change; Climate Control and possible Climate Change are strongly intertwined.
8. Implementation
A number of practical considerations must be made—the transformation of the ideal etendue conserving curve into a real optic will imply some loss in the theoretical efficiency.
8.1. Inter-Mirror Blocking
In reality the continuous curve of infinitesimal points is a series of pleated mirrors. This real optic will not conserve completely etendue as there will be a small amount of blocking between mirrors. We have developed a criteria to understand where there will be blocking:
These equations can be reduced to their dependencies on x and y, showing how the potential for inter-mirror blocking depends on their location relative to the receivers.
From inspection of the equations it is clear that blocking will be reduced as the vertical distance from the receivers, h−y, is increased. Therefore blocking will be a problem in the upper parts of the space. Furthermore, the product of the terms x and w−x will be greatest in the center of the space, which means that the center of the optic will be another potential problem area for inter-mirror blocking.
8.2. Sun Tracking
The solar roof must have some capacity to track the sun throughout the day—either through rotation of the mirrors or transformation of the curve. Structurally the second idea appears more complex, and so we consider the rotation of mirrors. The blocking criteria will become:
θz varies from −90° to 90°, and so it seems that it will alternately alleviate and exacerbate blocking.
The changes described above, and others, may be made in the solar canopy systems and methods described herein without departing from the scope hereof. It should thus be noted that the matter contained in the above description or shown in the accompanying drawings should be interpreted as illustrative and not in a limiting sense. The following claims are intended to cover all generic and specific features described herein, as well as all statements of the scope of the present method and system, which, as a matter of language, might be said to fall there between.
This application claims priority to U.S. Provisional Patent Application No. 61/488,928, filed 23 May 2011, which is incorporated herein by reference in its entirety.
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