With mass production of solar panels and industrial learning bringing the cost of solar electricity below 2¢/kWh in many parts of the world, the question is increasingly asked: what research is left to do in solar energy? Cheap photovoltaics with large-scale battery storage on the horizon, so the argument goes, are about to solve all of our energy problems, so why should we bother ourselves with anything else? The invisible hand of the market is now in control, and will guide us to a sustainable solar future. This manuscript seeks to cast a bit of doubt onto this looming consensus and argue that there is still work to be done in ensuring a maximally sustainable solar future. Drawing on ongoing projects on building-integrated photovoltaics, agricultural photovoltatics, and photovoltaic-thermal systems, we will demonstrate how the emerging PV paradigm falls short of its greenest possible form, and illustrate potential technological solutions based on solar concentration to more fully realize this potential.
As electricity from both photovoltaics and concentrating solar power has become dramatically more affordable in the last several years, the prospect of converting entire national electrical grids to run almost entirely on renewable energy has become feasible in any location with a strong solar resource. However, focusing only on the electric grid neglects the large portion of our energy usage which is thermal rather than electrical. One frequently proposed solution has been to simply electrify all thermal processes, for example in heavy industry, space and water heating. We suggest that the direct use of solar heat for thermal processes enables savings in terms of energy loss, land usage and dollar cost. Focusing on the requirements of heavy industry, desalination and long-distance heat transport, we identify the current and future potential of efficient collection, use and distribution of solar heat to extend the ongoing renewable revolution beyond the grid.
As the cost of electricity from photovoltaics drops rapidly, some have begun to ask whether solar concentration has any place at all in our energy future. Nevertheless, even in Dubai, where record-low costs for PV electricity has recently been achieved, the state utility DEWA, a combined power and water provider, recently ordered the construction of a 700MW CSP plant which will sell electricity at 7.3 c/kWh, more than double the cost of energy from PV! This premium is associated to the intrinsic energy storage ability of CSP systems as PV energy production profile does not fully match electricity demand curve. Clearly, CSP provides an added value that points a way forward for solar concentration technologies. While the capacity for energy storage is the critical factor in this situation, concentration-based approaches can be advantageous in other aspects of the energy-water nexus, especially where desalination is the dominant pathway to satisfy water demand. We will discuss several areas where solar concentration can provide benefits for electricity and water production, including solar-driven water desalination; integration of solar electricity, daylighting and heating capacity in buildings; and boosting capacity factors and LCOE of unconventional photovoltaic power systems.
The need to cool people in a warming world has led to renewed interest in radiative cooling in recent years. Most recent research has focused on the development of spectrum-selective materials designed to radiate in the atmospheric window while suppressing absorption of radiation outside of this window. However the alternative approach of using angular selectivity, via the inclusion of nonimaging optical components to restrict the cooling element’s field of view, has been neglected. Here we argue for the value of nonimaging optics in the design of practical radiative cooling systems.
Spectral splitting is widely employed as a way to divide light between different solar cells or processes to optimize energy conversion. Well-understood but less explored is the use of spectrum splitting or filtering to combat solar cell heating. This has impacts both on cell performance and on the surrounding environment. In this manuscript we explore the design of spectral filtering systems that can improve the thermal and power-conversion performance of commercial PV modules.
In designing solar concentrator optics there are many parameters that must be optimized in order to create a useful system, such as compactness, number of elements or interfaces, and acceptance angle, among others. Using geometric optics, tradeoffs between these parameters become inevitable. For example, a lens, trough or dish may be compact but has low tolerance of angular misalignment; angular tolerance can be improved by adding secondary and tertiary optics, but this increases complexity and reduces optical throughput; nonimaging optics such as the CPC offer wide acceptance angles from as single element, but are too long to be practical, in most applications, above low concentrations. These tradeoffs can be avoided by using angle-selective photonic materials to exploit the equivalence between angular restriction and concentration. Recently, broadband angular selectivity in optical films has been demonstrated by the Soljacic group in MIT. In this collaborative work we use this material to experimentally demonstrate two visible-spectrum optical concentrators. We demonstrate that these concentrators are thermodynamically ideal when the material properties are ideal, and describe the material improvements most essential for improving device performance, and discuss how commercial solar concentrator systems could be improved by the use of angular-selective optics
With PV module prices low and flat-plate technology dominating the market, the solar industry has taken a hard turn away from concentrator PV systems, which has faced high cost barriers. Despite this, there may still be space for CPV products and concepts to contribute, but these advantages are best realized by aggressively rethinking the way we design PV concentrators.
Optical spectrum splitting systems that divide light between independent solar cells of different band gaps have received increasing attention in recent years as an alternative to expensive multijunction cells for high-efficiency PV. Most research, however, has focused on dichroic filters and other photonic structures that are expensive to manufacture. This has the effect of transferring the cost of the system from the PV cells to the optics. As a low-cost spectrum splitting approach we designed a prismatic lens that simultaneously splits and concentrates light and can be fabricated by injection molding. We present experimental results of a two-cell demonstration system, and calculations for low-cost configurations of commercial solar cells, enabled by the removal of lattice-matching requirements.
We demonstrate 3D-printed nonimaging concentrators and propose a tracking integration scheme to reduce the external
tracking requirements of CPV modules. In the proposed system, internal sun tracking is achieved by rotation of the
mini-concentrators inside the module by small motors. We discuss the design principles employed in the development
of the system, experimentally evaluate the performance of the concentrator prototypes, and propose practical
modifications that may be made to improve on-site performance of the devices.
We present and analyze a design for a self-tracking solar concentrator based on a switchable-transparency optical element. The switchable element forms a moving aperture that tracks the motion of the sun to admit light into a CPC in which rays are 'recycled,' undergoing many passes through the concentrator to increase the absorption probability. This design has the benefit of not requiring any control of the angular profile of internal radiation, in contrast to other design that rely on total internal reflection to confine and transport the light. Via probabilistic models and rigorous ray tracing, we show that this design can exhibit performance comparable to other self-tracking designs. In particular we demonstrate a system with a 70x geometric concentration ratio and a tracking range of ±20°, achieving optical efficiencies of up to 65%.
We present a novel optical element that behaves as a dynamic aperture capable of tracking a moving light source. The
element is based on a composite material which when heated undergoes reversible transition from an opaque to
transparent state, resulting from a phase transition in one of its components that modifies the microstructure of the
material. The material has been designed to undergo a localized transparency transition at the point of illumination by a
focused beam, activated by the absorption and conversion to heat of a portion of the incident light. As a result of this
mechanism the aperture reactively tracks a moving light spot, such as that created by focusing sunlight onto a surface
during the sun’s apparent motion through the sky. Such an element has been proposed as a solution to the sun tracking
problem of solar concentration, as it allows admission of sunlight into a concentrating light trap over a wide range of
We present a design for a modification of a previously proposed light-trapping solar collector that enables reactive solar tracking by the incorporation of an optically activated transparency-switching material. The material forms an entry aperture whose position reactively varies to admit sunlight, which is focused to a point on the receiving surface by a lens or set of lenses, over a wide range of solar angles. An analytic model for assessing device performance based on statistical ray optics is described and confirmed by raytrace simulations on a model system.
High efficiency concentrator photovoltaic systems are currently based on costly III/V cells and, to offset the high cell capital cost, elevated optical concentrations are used, with consequent reduction in acceptance angles and tight tolerance optics. While this allows for spectacular conversion efficiencies, it does not provide cost effectiveness in a market dominated by low efficiency/low cost technologies. An alternative approach, well known in literature, is based on the combined use of an optical concentrator and a spectral splitting element allowing for the use of separate cells with different spectral responses and, thus, opening the way to a much wider range of possible materials and technologies. While many configurations have been presented during the years, optical efficiency has often been an issue due to the separate action of the concentrating and splitting element. We propose here, as substantial evolution of a previous design , a single injection molded plastic non-imaging optical element embodying both two axes concentration and spectral splitting functions. Based on the specific dispersion characteristics of polycarbonate and on a constructive analytical design procedure, this element allows for optical efficiencies exceeding 80%. Theory, simulations and preliminary experimental results will be presented.
In this article we discuss an emerging concept for non-mechanical solar tracking that can have a significant impact for the design of next generation concentrator photovoltaics systems. Based on the modification of the optical properties of the concentrator elements instead of their mechanical rearrangement, self-tracking concentrators, with recently demonstrated prototypes, could make the mechanical trackers redundant expanding the scope of application of CPV systems. We propose here a new approach to a reactive-tracking system, analyze its underlying physics and discuss initial experimental and simulation results towards the development of a prototype.