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Department of Engineering

Engineering energy roof: Generating electricity, generating ideas

Engineering energy roof: Generating electricity, generating ideas

The roof of the Inglis Building at the Department of Engineering

Faced with a leaky roof, the Department turned a routine maintenance job into an opportunity to generate its own electricity. The new 'energy roof' is now saving money and carbon, and has become a valuable resource for research and teaching in the Department.

What's really important is the ability to accurately model the potential of these kind of projects

David Green, Superintendent of the Engineering Workshops

The challenge

The first new building on the Department of Engineering's current site, the Inglis is typical of its era. Built in the 1920s, its sawtooth roof with north-facing window lights lets in the sun, but it also lets out the heat and, more recently, had begun to let in the rain.

Needing to replace the leaking roof, the Department also wanted to insulate it, so decided to turn a pressing but routine maintenance task into an opportunity for teaching and learning. They asked one of their undergraduates, Adam Booth, to use his summer placement to model the fabric of the building. His calculations revealed that insulation alone could deliver an energy saving of 46%.

What, then, could be achieved by adding solar photovoltaic (PV) panels at the same time as replacing and insulating the roof? Although the orientation and elevation of the Inglis roof's south-facing slopes are ideal for solar PV, the building also has a soaring brick chimney which acts as a giant sun dial, its long shadow revolving around the roof every sunny day.

The major challenge was working out which types of PV and in which configuration would be least affected by the chimney's shadow. “Shading matters because on certain panel orientations and certain types of inverter, if a shadow clips off one or two cells of a panel, the whole panel stops generating,” explained David Green, Superintendent of the Engineering Workshops. “We wanted a design that meant even if part of a panel was in shade, we could still generate some power.”

This challenge fell to another Engineering student, Caston Urayai, who, after finishing a PhD in the Electrical Power Conversion Group, used commercial software to model how much energy the roof might generate depending on the weather, the performance of different panels, panel design and the type of inverter.

Determined to apply as much of what they – and other parts of the University – were learning about saving energy, the Department then decided to ask Urayai to carry out further work to investigate opportunities for using the direct current (DC) the PV panels generate, rather than converting it first to AC.

“When you convert DC to AC you lose 10-20% of the power, so we are now looking at whether we can use the DC generated by the panel rather than going through an inverter,” said David. “This links us to work done in the Computer Laboratory, where Professor Ian Leslie has designed, built and installed DC light fittings that work this way.”

The solution

Based on what they learned from their students' detailed modelling, the Department opted for a PV panel produced by Viridian Solar. The decision was based on performance, but the choice also pleased Green because of the firm's close links with Cambridge. “Viridian employs some of our graduates, and they were very keen to work with us to explore opportunities for research,” he said.

Installed in 2013, the energy roof has a mixture of string and micro-inverters. The micro-inverters generate slightly less power but are better at generating under shadow-cast conditions, so for parts of the roof where there is good solar gain for only part of the day, micro-inverters offered the best solution.

Like the PV panel producer's Cambridge connections, the micro-inverters, produced by Enecsys, also build on research done in he Engineering Department. According to David: “The micro-inverter includes thin-film technology that our Electrical Engineering Division's been involved in, so there's a nice link between our teaching and research and the commercial products that have benefited from the work we've done here.”

Building on the success of phase 1 of the energy roof, the Department moved on to phase 2 – a second energy roof above the new Dyson Design Centre that will be established in 2015. Based on a similar installation of insulation topped by PV and a similar process of modelling the potential energy generation, phase 2 is also feeding into the Department's desire to maximise efficiency by using DC.

“We are looking at a multiplex power supply so that energy generated from the PV can meet demand for lighting etc, but if we're generating energy from the PV but don't have a need for lighting we are going to store it in batteries. Then when we need lighting we can use power from the PV, batteries or the grid,” David Green explains.

Going one step further, the department is working with Luxonic Lighting, a firm that manufactures commercial light fittings, to develop a DC version, which will be able to use power from phase 2 of the energy roof without conversion to AC. “If we're successful, we can look at extended use of DC, for example in the Engineering Department’s new James Dyson Building, which could be a DC-lit building,” he says.

The impact

In the year since it was installed, phase 1 of the energy roof on the Inglis Building generated 82,471 kWh. While it represents only a small proportion of the department's energy consumption, Departmental Safety Officer Ian Slack thinks this needs to be set in context.

“We are a large department and we are very energy hungry,” he explains. “In July 2013 the panels generated 8,658 kWh. Although this equates to only 3.1% of our energy consumption, it's a reasonable amount of energy, which in an administrative or an arts and humanities building would represent a good chunk of their energy needs.”

As well as saving energy and carbon, the energy roof is providing important opportunities for the Department's teaching and research, which in turn are helping inform decisions about similar projects elsewhere in the University.

“What's really important is the ability to accurately model or calculate the potential of these kind of projects,” says David Green. “Everyone will try and sell you PV, but modelling is crucial to getting an idea of how long the payback period will be. And although the level of any future government feed-in tariff is an unknown, when you start considering using DC you can reduce that uncertainty because it's unaffected by future changes in the tariff.”

Making the most of the need for modelling, the Department has been able to offer several students real-world problems to solve. It is using the energy roof in teaching its renewable energy course for third year undergraduates, thanks to the University's Living Laboratory project which funded the software they needed.

According to David: “In the past students have used a mocked-up panel on the roof, but now they can model a professional installation. Linking the course directly to our energy roof means our undergraduates are learning from a real installation, not a mock up.”

As well as modelling from the energy roof, by working closely with Estate Management, students have also modelled the potential gains from PV on other University buildings. “At the end of the course, each student group presented its findings to Estate Management. If their modelling showed good returns, our students could be contributing to the next generation of energy roofs at Cambridge,” he says.

This article originally appeared on the Cambridge Green Challenge website.

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