The Integrated Concentrating (IC) Solar Facade System is a building integrated photovoltaic system that takes a dramatically different approach than existing building integrated photovoltaic (BIPV) or concentrating PV technologies to provide electrical power, thermal energy, enhanced daylighting and reduced solar gain. The system (for both retrofit applications and new construction) is architecturally integrated into the facades and roof atria of buildings while still providing maximum outside views and diffuse daylight for the building users. These benefits are accomplished by miniaturizing and distributing the essential components of concentrating PV technology within the weather-sealed windows of the building envelopes. (An alternative approach is to place the components behind the external façade envelope and construct an inner surface to protect the mechanisms.) The IC Solar System produces electricity with a PV cell, captures much of the remaining solar energy as heat for domestic hot water, space heating (or, possibly, for distributed absorption refrigeration cooling), reduces solar gain by the building, and enhances interior daylighting quality, thus reducing overuse of artificial lighting. The design and operation of the system permits direct partial views of the outside by the building's inhabitants. The modular design can be attached to a range of existing building structures or implemented into new designs. The tracking IC Solar Module System has been demonstrated in several 'proof of concept' lab-scale prototypes with multiple cell types.
The technical challenges of the IC Solar System are to produce a low-cost shading system for windows that:
1. uses as much of incoming direct normal irradiation as possible in the production of electricity
2. allows as much diffuse incident irradiation as possible to enter occupied spaces for day-lighting
3. requires little maintenance
4. captures, as thermal power, that which is not directly converted to electric power via the PV cell, thereby lowering building cooling loads
5. is aesthetically attractive for architectural markets
The IC Module has thus far been designed to effectively use the direct solar irradiance incident on the surface(s) of a building to augment or power the building. This irradiance, after initial reflections at the air/glass/air interfaces on the exterior of the building, has been transmitted to a faceted type lens. The lens directly concentrates (>400:1) the light onto a high-efficiency multijunction PV cell, recently demonstrated at 39.4% under 411 suns. The power not converted to electricity is captured via a coolant flow through the receiver on which the cell is mounted. This coolant is proposed for hot water heating, space heating (if needed), or (potentially) for absorption refrigeration cycles. With a high concentration ratio and small PV cell, the size of the modules dictates the allowable two-axis tracking error. For zero loss of direct irradiance on the cell, 900 µrad tracking error tolerance is allowed. Through iterative modeling, a planar lens shape in a close packed array was determined to ensure maximum conversion of solar energy to electrical power while permitting substantial day-lighting. The current prototype, a turntable type achieves a maximum combined tracking error of less than 800 rad. The modules and tracking mechanism are environmentally shielded from external forces, such as direct wind loading, by the exterior glass facade. Therefore, precise tracking can be achieved through inexpensive motors.
Because concentrating PV requires that the system track the sun, we integrate the CPV technology into a dynamic shading system, whereby the windows (required on the buildings whether or not our system is enclosed by them) protect the system from weather and wind loading. This alleviates a major obstacle in the application of previous concentrating PV systems (usually large-scale trackers) that have not been able to viably accomplish tracking accuracy requirements with wind loads and maintenance schedules. Therefore, as a distributed system, the IC Solar Facade system capitalizes on existing building structures as scaffolding and protection for the system, thus reducing cost for an expensive tracking structure and encasement and eliminating the need to transport the power far from its generation location.
Simple payback estimates indicate dramatically reduced payback periods in comparison to conventional PV systems currently on the market, while substantially increasing efficiency by orders of magnitude.
