CASE STUDY | ST JOHNS COLLEGE, OXFORD, LIBRARY AND STUDY CENTRE In winter, space temperatures in the main library are kept between 19C and 21C The amount of glazing in the building means lighting should not be on during the day boreholes, each 55m deep, sunk beneath the Great Lawn. (See panel, The Great Lawns ground source heat pumps). Unsurprisingly, the buildings temperature and humidity are carefully monitored, with its temperature maintained between 12oC and 15oC. The space is fitted with mechanical ventilation and local dehumidifier units, but the space is largely passive in operation, without the need for intensive servicing. The only reason wed need to heat the space would be if the humidity increased at low temperatures, says Rushford. The archive air supply does have a cooling system, as does a small IT server room. There is a fan coil fitted with a cooling coil (archive) and fan coil unit (server room) connected directly to the borehole array not via the heat pumps because the ground is roughly at the right temperature to cool the air if needed, says Rushford. This solution has the additional benefit of slightly increasing the temperature of the borehole array and, thereby, the efficiency of the heat pumps when providing heating. In summer, when heat is not required, the warmed water is returned to the ground to help recharge ground temperature. All domestic hot water is supplied by instantaneous electric hot water heaters. Photovoltaic (PV) panels installed on the buildings roof help meet the buildings electrical demand although energy modelling at the projects outset confirmed that space on the roof for PVs was insufficient to meet the buildings electrical needs. At the early stages, we did quite a lot of hand calculations to see, very broadly, how much energy we expected it to use. Then, as we went through into more detailed design, we used CIBSE TM54 methodology to evaluate how much operational energy the building might use, which outlined a number of different scenarios, that impact what the consumption might be, says Tom McNeil, senior engineer and building performance leader at Max Fordham. The on-building PVs generate a maximum of 42kWp. The evaluation showed that, with the limited amount of PV that could be squeezed on to the roof, it was not going to achieve net zero carbon, McNeil explains. Nevertheless, since the building has started to be used by students, Max Fordham has been working to optimise its performance to minimise energy use. The project specification drawn up by Max Fordham has helped THE GREAT LAWNS GROUND SOURCE HEAT PUMPS The college had concerns that changes to the ground temperature, caused by the borehole installation, could affect its historic lawn. In response, Max Fordham carried out an analysis to prove the arrays impact on the ground temperature beneath the lawn would be negligible, now and in the future, once solar radiation absorbed by the lawn was factored into the energy balance. As well as heat to temper the reading rooms fresh air supply, the GSHPs provide all the space heating. In most areas, this is through an underoor system enhanced by trench heaters in places with higher heat losses, which helps prevent downdraughts where the faade is glazed. The heating is weather compensated. Weve been tweaking it with the control specialist to make sure the water is hot enough in the winter to meet the heat demand but, equally, we want to keep the temperature as low as possible to get the best GSHP efciency, says McNeil . Generally, the heating is maintained at around 40C; it only needs to go above that when it gets very cold outside. The approach appears to be working: the rst year of monitoring the heat pumps demonstrated a coefcient of performance of 3.6. Heating is also supplied to the basement archive room, although Rushford says it has not yet been needed. The archive room contains the colleges collection of rare books and manuscripts, including material from alumni of St Johns such as Robert Graves and Philip Larkin. 22 April 2022 www.cibsejournal.com CIBSE Apr 22 pp20-22, 24 St Johns College.indd 22 25/03/2022 17:33