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Optimal Utilisation of the Electricity from Amorphous-Si Photovoltaics
in Commercial Buildings

Michael M.D. Ross
Helsinki University of Technology

Full Text of Article
Link to Helsinki University of Technology (TKK)
Link to Advanced Energy Systems Group at TKK

Note on Authorship:

This article was authored by Michael M.D. Ross, principal of RER Renewable Energy Research, while he was an employee of the Advanced Energy Systems Group, Department of Engineering Physics and Mathematics, Helsinki University of Technology.

Citation:

Ross, Michael. Optimal Utilisation of the Electricity from Amorphous-Si Photovoltaics in Commercial Buildings. Report # EU-JOULE JOR3-CT96-0096, ASICOM Programme, Helsinki University of Technology, Otaniemi, Finland, June 1999, 94 pp.

Summary:

Photovoltaic (PV) output and electricity consumption were compared for a medium-sized office building using a mathematical model. In all but the smallest commercial buildings, loads during occupied hours will be so large that all electricity produced by an amorphous silicon (a-Si) PV array can be used within the building. This simplifies utility interconnection and avoids problems of low utility buy-back rates.

In a typical AC busbar system, optimizing the use of photovoltaic electricity largely means choosing and sizing the inverter appropriately. Module, string and central inverters are compared; reliability issues are discussed. The optimal inverter will have a rated capacity of 45 to 75% of the array in most cases. Optimal array design and approaches that minimize the cost of wiring, diodes, and other minor components are also discussed.

While PV's DC output is typically tied to the AC building grid with an inverter, in many commercial buildings it could be used for large DC loads. These include uninterruptible power supplies, variable speed drives for ventilation and cooling systems, fluorescent lighting, computers, and power conditioners. Interconnection topologies are explored; in many cases both the inverter and rectifier of a typical AC/DC busbar configuration can be eliminated. This reduces costs, losses, and reliability problems, and leads to systems that are up to 38% more cost-effective than typical AC busbar systems.

Marginal cost tariffs affect the value of PV power systems. American studies show that on summer peaking utilities, PV generates both energy and capacity benefits, which can lead to demand charge savings for commercial buildings. Some of these may be realized even on winter peaking utilities. Distributed photovoltaic generation may generate additional benefits in the local transmission and distribution system.

Demand and capacity benefits can be increased using a buffer. Although it matters little whether a buffer is charged by the grid or a PV system, there is an important synergy between the buffer and PV, which increases the benefit realizable with a unit of buffering capacity. Rigorous approaches for optimal sizing of PV and buffer are suggested; these use the cost of capital in conjunction with the declining marginal benefits of demand reduction and waste storage as the array or buffer size increases. In the case of optimal buffer sizing, the approach is complicated by requirements for load, climate, building demand, and utility price data; although not especially practical, the approach does clarify the economics of buffering. As practical buffering technologies, batteries are more familiar to PV system engineers, but cool storage seems nearly ideal from an engineering and economic point of view, at least in buildings on summer peaking utilities.