Great St Mary's Church

Great St Mary's Church

A remarkable, historical site

At the heart of Cambridge University stands Great St. Mary’s Church.  The present-day structure was constructed between 1478 and 1519, but there was likely a structure on the site as early as 1205.  This ancient church formed part of the first roots of Cambridge University and the town at large, with the church deemed the official meeting place of the University for several centuries until 1730.  It is now the official University Church, and students are required to live within 3 miles of its tower.

The church has an active congregation that is centrally engaged in many facets of local life.  They are community leaders on many fronts, including environmental progress and sustainability, so when the need arose to replace a south-facing lead roof, the Parochial Church Council (PCC) sought quotes for a solar PV installation.  Cambridge Solar Ltd provided our careful analysis of the site and its generation potential, and was awarded the contract to perform the installation on the basis of our clear, objective energy performance predictions, and our demonstrated attention to detail in sensitive design and construction circumstances.

Environmental impact

Positive environmental impact was a core motivation for the PCC to install solar panels, so Cambridge Solar Ltd produced the standard MCS prediction of panel performance, as well as our best estimate of their usage.  We took the time to listen carefully to the description of the church’s energy needs, and to explain to them strategies for maximizing their self-consumption of energy produced by the PV system. The system is expected to generate over 10,000 kWh per year, so assuming a conservative 40% self-consumption, and 290 g CO2/kWh of grid electricity, the church will prevent the emission of over 23,000 kg CO2 over the next 20 years.

The church and its congregation take a long view on their role in society and the local community, so making a commitment to a low-CO2 energy source was a clear choice.  While the reliable financial return is a boon, the congregation takes genuine pride in their role as community leaders on environmentalism, so generating their own renewable energy was a top priority as a meaningful demonstration of the church’s goals.

Financial impact

Based on the PCC’s own reasonable estimate of their ability to self-consume the solar energy generated, the PV system should have a payback time of less than half its warrantied lifetime.  The solar PV will therefore be a solid, remunerative financial investment for the church, which has a limited number of other options for energy efficiency improvements (like insulation, or a ground-source heat pump) given the architecture and location.

Novel mounting for architectural integration

Great St. Mary’s is a Grade I-listed historical building, the same level as Westminster Palace in London, so the design of the PV system had to be, above all else, unobtrusive to the celebrated architecture, and enhancing to the aesthetic if at all possible.  Beyond this overarching aesthetic demand, the particulars of the mounting system had to be integrated structurally with the new lead roof. Lead is a rather malleable metal subject to more thermal expansion than other typical roof materials---nearly twice that of copper, and nearly three times that of concrete.  Beneath the lead is a wooden roof that must be protected from water damage.

Cambridge Solar Ltd worked within these parameters to design and produce a new, self-sealing mounting attachment that creates a water-tight interface with the soft lead surface when it is screwed into the wood beneath.  When the lead expands or contracts, the mounts will still be sealed against water that would cause major damage to the structural wood supports. The mounting attachments are spaced to accommodate the structural lead “rolls” parallel to the slope of the roof, as well as a new man-safe anchor system for future work.  These structural elements ensure that the system will be secure for its warrantied lifetime and beyond, since scaffolding to the roof is expensive and disruptive.

Aesthetically, the all-black panels are hidden behind a short parapet such that they are invisible from street level.  They are not noticeable to anyone inside the church unless one stands on the upper mezzanine balcony and looks down. Thus Cambridge Solar Ltd has helped Great St. Mary’s retain its architectural splendor while utilizing the latest technology to advance its financial stability, and environmental preservation goals.