SEA Solar Energy Conversion System
Cut-Away System View Illustrating Sun Tracking
SEA Solar Energy Conversion System
The SEA solar energy conversion system is a cogeneration system, producing both electricity and valuable low-grade thermal energy (material temperatures less than roughly 80 to 90°C). Our system has multiple rows of parabolic trough reflectors that focus solar radiation onto actively cooled arrays of solar cells. The solar cells convert a portion of the incident radiation directly into electricity. The portion that is not converted to electricity is collected as low-grade thermal energy in coolant flowing through cooling channels that support the solar cells. This low-grade thermal energy is normally discarded as waste heat, but SEA systems collect it, store it, and make it available for later use.
The trough reflectors used in solar energy conversion systems track the sun by rotating in one angular dimension. Sun tracking for an SEA system’s reflectors is accomplished by rotating the entire system in a horizontal plane about a central vertical axis, with the optical planes of the reflectors continuously maintained in a vertical orientation. As the system rotates, the reflectors and the reflector support structure are subjected to forces that are constant in both magnitude and direction. This greatly simplifies the mechanical design of the system, reduces fabrication and installation costs, and increases system reliability. Also, the reflectors longitudinal axes can be rotated into alignment with wind direction whenever wind velocities reach a high level, thereby significantly reducing the possibility of storm induced damage.
Since the reflectors in an SEA system don’t tilt or roll as in previously deployed trough reflector systems, they don’t produce mutual shadowing at low sun angles. That’s why our system’s reflectors can be placed side by side, collecting sunlight from more than 95% of the area covered by the reflector field. Efficient use of land area means that SEA systems can be deployed near the energy markets they serve, utilizing the solar resource locally, even in locations where available land may be expensive or in limited supply.
When irradiated by the intense sunlight (25 to 30 suns) coming from the SEA system’s parabolic trough reflectors, the solar cells (back-contact, single-crystal silicon solar cells) can produce electrical energy with an efficiency approaching 25%. Active cooling of the cells maintains their high efficiency even when they are exposed to highly concentrated sunlight. Also, the light-concentrating power of the parabolic trough reflectors reduces — by a factor of 25 to 30 — the area, and thus the cost, of the solar cells required to generate a given amount of electrical power. Just think about that. The cost of the solar cells in an SEA system is only 3 to 4% of the cost of the cells in a non-focusing system with the same output power.
In summary, SEA solar energy conversion systems are modular, scalable, mechanically simple, and they make very efficient use of both available land area and incident solar radiation. They collect light from more than 95% of the land area covered by the reflector field and they convert more than 90% of incident direct solar radiation into useful output energy.
These are remarkable numbers, but of even greater importance is the fact that our SEA systems can be integrated with proven technologies that provide long-term (seasonal) storage of both electrical and low-grade thermal energy.
The SEA solar energy conversion system is a cogeneration system, producing both electricity and valuable low-grade thermal energy. More than 90% of incident direct sunlight is converted to useful output energy.
Because of the way sun tracking is done in SEA systems, they are mechanically simple, they have low fabrication and assembly costs, they have high degree of reliability, and they are very resistant to wind-induced damage.
SEA systems collect sunlight from more than 95% of the area covered by the reflector field. This means that they can be deployed near the energy markets they serve, even in locations where available land may be expensive or in limited supply.
SEA systems reduce by a factor of 25 to 30 the area, and thus the cost, of the solar cells required to generate a given amount of electrical power. The cost of solar cells in an SEA system is only 3 to 4% of the cost of the cells in a non-focusing solar-cell-based system with the same output power.
SEA solar energy conversion systems are modular, scalable, mechanically simple, and they make very efficient use of both available land area and incident solar radiation.
A Quick Look at How We Use Energy
Our modern societies get the energy they need by converting primary energy resources (sources of energy that exist in nature, such as coal, oil, natural gas, wind, sunlight, hydro, etc.) into energy carriers (forms of energy that we actually use, such as electricity, various fuels, and low-grade thermal energy). One of the most surprising things about our modern industrial societies is that more than 75% of total domestic energy usage, and nearly 40% of domestic electricity usage, is for low-grade thermal applications. The charts below give a detailed breakdown for the United States in 2016. (Similar but somewhat smaller percentages exist for other countries and for other economic sectors.) A large part of the low-grade thermal energy currently used in these applications is provided either directly by burning fossil fuels or indirectly by using electricity that is generated from fossil fuels. This energy could be provided by the “waste heat” collected and stored by SEA cogeneration systems, thus saving consumers a considerable amount of money and significantly decreasing the amount of electricity that we need to generated to keep our societies functioning.
2016 Residential Energy and Electricity Use (Source: US EIA)Long-Term Storage of Low-Grade Thermal Energy
Long-Term Storage of Low-Grade Thermal Energy
In SEA solar energy conversion systems, heated coolant exiting the cooling channels that support the solar cell arrays can be circulated through borehole heat exchangers, where thermal energy in the coolant is transferred to underground thermal storage reservoirs (undisturbed layers of soil, gravel, and rock). This energy can be reclaimed as needed — after days, weeks or even months of storage — for end-use in applications shown in the charts above, or it can be used as process heat in industrial or agricultural operations. This proven technology is referred to as Borehole Seasonal Thermal Energy Storage (BSTES) and it is now being used successfully in a number of locations in northern Europe and Canada. A good example of the BSTES technology is found at www.dlsc.ca. The technology used in these systems is extremely efficient, with coefficient of performance (COP) values of 30 being typical. For every joule of energy used in operating these systems, 30 joules of energy are delivered to the user. That’s why we shouldn’t think of low-grade thermal energy as being of low value. The true value of the low-grade thermal energy collected and stored by SEA systems is measured in terms of the cost of the electricity and the fossil fuels that are displaced by its use.
Long-Term Storage of Electrical Energy
Regarding the electricity produced by our SEA systems, some will be used as it is produced, some will be stored in batteries for the short term (overnight or for a few days), and the majority will be stored for the long term (season-to-season) in the form of various fuels. The fuels, produced through a series of processes that begin with the electrolysis of water, can be stored until needed and then burned in heat engines to regenerate a portion of the electricity that was originally used to create the fuels. The fuels can also be used to perform other tasks, such as powering transportation vehicles.
In the past, processes that involved electrolysis, a fuel, and a heat engine — Electrolysis-Storage-Reconversion cycles (ESR cycles) — could not be considered viable energy storage technologies, primarily because of inefficiencies that were inherent in electrolysis processes and heat engines. At SEA, we have designed and will soon begin development of a high-efficiency, two-stroke internal combustion engine. With this engine, ESR cycles have become cost-effective energy storage mechanisms, and long-term storage, the last impediment to wide-spread utilization of solar energy, has been removed.
EA has patented a high-efficiency two-stroke internal combustion engine which makes Electrolysis-Storage-Reconversion (ESR) cycles economically viable options for long term storage of electrical energy.
Features of the SEA Solar energy Conversion System
Parabolic trough reflector systems track the sun by continuously changing the orientation of the reflectors in one angular dimension. The technique used for sun tracking in SEA systems is different from the sun tracking in other parabolic trough reflector systems, and that difference gives SEA systems several important structural and performance advantages. To understand why sun tracking techniques are so important, let’s look first at existing parabolic trough reflector systems.
All currently operating parabolic trough reflector systems, both thermal and solar-cell-based, have reflectors whose longitudinal axes are permanently aligned in a fixed direction, usually in either a north–south direction or an east–west direction. In these systems, each reflector tilts (rolls) about an axis parallel to its focal line as it tracks the sun. Systems with this type of sun tracking are referred to as Tilting Trough Reflector (TTR) systems. TTR systems are by far the most common type of concentrating Concentrating Solar Power (CSP) system currently in use, but they have three serious problems.
The first problem with TTR systems is that, since the reflectors tilt in order to track the sun, the individual rows of reflectors must be separated in order to avoid mutual shadowing at low sun angles. This leads to poor land area utilization, which, at high sun angles, is often less than 30%. (The situation is even worse for Central Tower systems because the reflectors track in two angular dimensions, rather than just one, and land area usage within the reflector field is often less than 20%.) Poor land area utilization means that TTR systems can only be deployed in areas where average daily insolation is high and land costs are low. Such locations are usually far from the population and industrial centers that use the energy produced. Since low grade thermal energy cannot be economically transported over long distances, the poor land area utilization exhibited by TTR systems prevents them from being used as cogeneration systems.
At high sun angles, when incident sunlight is most intense, a Tilting Trough Reflector (TTR) system collects solar energy from less than 30% of the land area occupied by its reflector field.
The second problem with TTR systems is that, during sun-tracking operations, the parabolic trough reflectors are subjected to directionally-varying forces. Reflector support structures are thereby exposed to severe bending and torsional stresses. Both the reflectors and the support hardware must be designed to function effectively in spite of these challenging mechanical requirements. This significantly increases TTR system cost, decreases system reliability, and seriously limits available options for optical design.
The reflectors in TTR systems require substantial mechanical support structure to withstand deformation stresses developed as the reflectors tilt to track the sun.
The third problem with TTR systems is that, since the parabolic trough reflectors are permanently oriented in a fixed direction, they are exposed to severe wind-induced stresses whenever strong winds blow crosswise to the direction of their longitudinal axes. This places additional mechanical demands on both the reflectors and the support structure, again adding to hardware manufacturing costs and system complexity, while at the same time decreasing system reliability.
SEA solar energy conversion systems use parabolic trough reflectors, but they track the sun in a way that avoids the inherent problems associated with TTR systems. In SEA systems, the rows of parabolic trough reflectors do not have a fixed north-south or east-west orientation. Instead, sun tracking is accomplished by rotating all the rows of reflectors and the associated rows of receiver elements as a single interconnected unit, with the rotation of the unit occurring in a horizontal plane about a central vertical axis. The individual reflectors do not tilt as they track the sun and the optical plane of each reflector is always held in a fixed vertical orientation. Since the reflectors don‘t tilt, they can be positioned side by side, thus providing efficient utilization of available land area. This solves the first problem mentioned above. also, since the optical planes of the SEA systems collect energy from more than 95% of the land area covered by the reflector field.
SEA systems also eliminate the third problem mentioned above, the problem of damaging crosswinds. Since the orientation of the rows of reflectors is not fixed in one specific direction, sun tracking for our systems may be temporarily interrupted whenever wind velocities reach a high level. The reflectors can then be reoriented so their longitudinal axes are aligned with wind direction. This feature greatly reduces the likelihood that mechanical stresses induced by crosswinds could cause physical damage to the reflectors or the support structure.
Since the reflectors in SEA systems don’t tilt as they track the sun, their focal length to width ratio (F/W) can be chosen independently of torque or mechanical stress considerations, and can instead be chosen so as to achieve other system goals, such as improving overall system alignment stability and reducing Fresnel reflection losses that occur at the front faces of the solar cell arrays. That’s what we’ve done in designing the SEA system, where F/W ratios are on the order of 0.6, with Fresnel losses at the solar cell surfaces of only a few per cent. In TTR thermal systems, F/W ratios are typically on the order of 0.25 to 0.35, with optical losses at the receiver that are typically on the order of 20%. (Part of this loss is the result of the lower F/W ratio and part is the result of the fact that the receiver is a circular cylinder instead of a planar surface.) One other advantage of larger F/W ratios is that, for a given reflector width, W, the uniformity of the solar image is improved and this increases the efficiency of the solar cells.
The fixed vertical alignment of SEA reflectors allows for longer focal length-to-width ratios, which reduce Fresnel losses at the solar cell surfaces and provides the capability for profile shaping of the solar image.
Because the reflectors in SEA systems are exposed to gravitational forces that are always parallel to the reflectors’ optical axes, free-hanging (edge supported) reflectors made from flexible rectangular sheets can be used to form the reflector surfaces. The reflectors in SEA systems have mathematically defined variations of transverse thickness (US Patent 10,001,297) that cause them to hang as parabolic troughs when properly supported at their edges. Since the reflectors in an SEA system are flexible and are not supported by the rigid structural members required in TTR systems, accurate optical alignment can be built into the structural framework supporting the reflectors. The resulting alignment is relatively insensitive to crosswinds and to irregularities in the elevation of the surface over which the system rotates as it tracks the sun. SEA reflectors and associated support structure can be economically manufactured by using simple, low-cost fabrication processes. Material costs are greatly reduced compared to TTR or Central Tower systems.
SEA’s patented free-hanging reflector design requires minimal support structure. The reflectors can be fabricated by using simple manufacturing techniques and since they are flexible, their optical alignment is relatively insensitive to crosswinds and irregularities in the surface over which the system rotates.
Each receiver assembly in an SEA solar energy conversion system consist of a planar array of solar cells mounted on the flat wall of a cooling channel. The system’s structural framework holds each array so that it is perpendicular to, and bisected by, the optical plane of the associated parabolic trough reflector, thus allowing the reflectors to form a planar image of the sun on the solar cell arrays. The maximum concentration ratio for parabolic trough reflectors (ratio of parabolic trough reflector width to solar image width) is 107. In SEA systems, the solar cell arrays are held near, but not in, the reflectors’ focal planes. This defocusing of the system provides control over the concentration ratio, which is set in the range of 25 to 30 in order to decrease the thermal impedance between the solar cells and the cooling fluid.
The SEA system is slightly defocused (25 to 30 concentration ratio) in order to increase the width of the solar image at the surface of the solar cells. This decreases the thermal impedance between the cells and the cooling fluid.
SEA solar energy conversion systems have been designed to operate year round, with energy storage capabilities bridging the seasonal variations of the solar resource. When integrated with proven long-term storage technologies, our systems can meet the needs of the energy markets they serve without fossil fuel or nuclear backup, even if the energy markets are located in regions where latitude or climate significantly decrease available solar radiation.
In summary, SEA systems are simple, efficient cogeneration units. The solar cells in SEA systems will produce electricity with efficiencies that are considerably better than the efficiencies of CSP thermal systems having steam-cycle generators, where the overall system efficiencies are typically down in the mid-teens. Like other solar-cell-based systems, SEA systems are scalable, that is, their efficiencies are independent of system size, which means they can be sized for, and located near, the energy markets they serve, thereby utilizing the locally available solar resource to produce energy that can be used locally. SEA systems also provide large quantities of valuable low-grade thermal energy which can be stored by means of proven, highly efficient BSTES technology.