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CASE STUDY | ONE ANGEL SQUARE FIT-FOR-PURPOSE PVS The roof-mounted PV array was selected as the most suitable installation because of building orientation and a favourable payback period. Project budget pressures meant alternative means of funding the PV array were explored and local authority partner organisations were approached. One of these provided the capital investment for a 1,200m2 photovoltaic installation on the unshaded at-roof areas of the building. he partner organisation benefits from the eed-in ariff i , but One Angel Square enjoys onsite energy generation. Whats more, the PV array meets the entire electrical load of the building during summer weekends and delivers an annual operational saving of more than 15,000 for Northamptonshire County Council and a i contribution to the investor of more than , per year. few services at high level in the offices. This gives a minimal aesthetic, but more importantly the concrete soffits provide exposed thermal mass to help absorb daytime heat. Night-time cooling recharges the soffits ready for the next day. The solution has the advantage of extending the period that natural ventilation can be used before the demand-controlled ventilation system comes into operation. If you can take out whole systems, you get big value-engineering savings, says Hepburn. With the chilled beams gone, BDP to help control solar gains boosted displacement airflows to the central floor plates and switched the perimeter trench heating to a four-pipe heating and cooling system, served by roof-mounted, gas-fired condensing boilers and lowenergy chillers. BDP modelling proved the operational energy benefits of retaining the mixed mode/demand-controlled ventilation approach for the scheme. Our TM54 modelling and sensitivity analysis estimated a fan energy saving of 30% by retaining the mixed-mode solution, says Hepburn. It is difficult to quantify the impact of this reduction in the actual building, but it is noticeable when visiting that the users enjoy the natural ventilation for a significant part of the year. TM54 modelling was used to estimate the buildings high, medium and low annual energy use: the figures for these were 112kWh.m-2, 90kWh.m-2 and 81kWh.m-2 respectively. Our targets align with a good-practice, naturally ventilated, small cellular office and are half that of the benchmark for a general office in TM46, says Hepburn. Our first year of energy-consumption data suggests the building is currently exceeding, slightly, the upper end of the TM54 prediction, at around 130kWh.m-2 per year but this is improving steadily through proactive engagement by BDP with the client and contractor teams, and regular site visits as the buildings settles down. The building also incorporates extensive smart (automatic) metering of electricity. including generation and export meters for the onsite electrical generation from the 1,200m2 roof-mounted PV array, after the feasibility of various types of renewable and low and zero carbon technologies were assessed (see panel, Fit-for-purpose PVs). In their citation for the award of Project of the Year Commercial/Industrial, the CIBSE judges described the scheme as a good-quality design, delivered within a limited budget. In particular, they liked the use of TM54 modelling to develop robust operational cost estimates to maintain value by defending engineering options in the design that would lead to better value outcomes. CJ 36 October 2019 www.cibsejournal.com CIBSE Oct19 pp32-34, 36 One Angel Square.indd 36 20/09/2019 16:52