ABSTRACT
In this paper we present, demonstrate and validate a method for predicting city-wide electricity gains from
photovoltaic panels based on detailed geometric urban massing models combined with Daysim-based hourly
irradiation simulations, typical meteorological year climactic data and hourly calculated rooftop temperatures. The
resulting data can be combined with online mapping technologies and search engines as well as a financial module
that provides building owners interested in installing a photovoltaic system on their rooftop with meaningful data
regarding spatial placement, system size, installation costs and financial payback. As a proof of concept, a
photovoltaic potential map for the city of Cambridge, Massachusetts, USA, consisting of over 17,000 rooftops has
been implemented as of September 2012.
The new method constitutes the first linking of increasingly available GIS and LiDAR urban datasets with the
validated building performance simulation engine Daysim, thus-far used primarily at the scale of individual
buildings or small urban neighborhoods. A comparison of the new method with its predecessors reveals significant
benefits as it produces hourly point irradiation data, supports better geometric accuracy, considers reflections from
neareby urban context and uses predicted rooftop temperatures to calculate hourly PV efficiency. A validation study
of measured and simulated electricity yields from two rooftop PV installations in Cambridge shows that the new
method is able to predict annual electricity gains within 3.6 to 5.3% of measured production when calibrating for
measured weather data. This predicted annual error using the new method is shown to be less than the variance
which can be expected from climactic variation between years. Furthermore, because the new method generates
hourly data, it can be applied to peak load mitigation studies at the urban level. This study also compares predicted
monthly energy yields using the new method to those of preceding methods for the two validated test installations
and on an annual basis for ten buildings selected randomly from the Cambridge dataset