Eco Measures Prove Themselves in 1st Year

“At last, an eco house that’s actually warm”, in the words of an early visitor with experience of eco buildings.

Since the air source heating system was commissioned in February 2014, the offices have been heated to a cosy 21oC, providing its users with a comfortable average temperature for the building of nearly20oC, with little variation between rooms. Air-tightness measures employed in the build ensure there are no cold draughts. The lift in temperature over the outside has ranged from zero in the summer months to 14.5oC in January 2015, with a full year average at 7.5oC.

The first year’s energy performance showed that …

  • Energy purchases for the retrofitted offices were reduced by over 89% from pre-retrofit figures.
  • Office energy purchases worked out at 19.5 kilowatt hours (kwh) per square metre per year. This was achieved with an average of 12.3 kwh of electricity purchased each day. Of this, 7 kwh was used at night, mainly to operate the air source heat pump (heating), and 5.3 kwh was used during the day for lighting, office, kitchen and therapy equipment. Heating the offices required only 11.1 kwh per square metre per year, well within the Passivhaus limit of 15 kwh.
  • Over the year, that 7 kwh of night time electricity brought an average of 20.3 kwh of heat into the building, giving a coefficient of performance (the ratio of heat generated to the power used to run the heat pump’s fans, pumps and compressor) in excess of 2.92. This figure is probably conservative, given that some of that 7 kwh of night time electricity was used for non-heating purposes, such as monitoring systems and lighting. At a cost of 8.631p/kwh for night time electricity, that gives a cost per unit of heat of 2.956p, 30% cheaper than gas (4.2p/kwh). Adding in the non-domestic Renewable Heat Incentive payment of 2.54p/kwh brings the cost comes down to 0.416p per unit, making the cost of heating the Eco Offices virtually free.
  • Energy purchases for the apartment (office space before the retrofit) were reduced by a less impressive 72%, but the smaller improvement needs to be seen in the context of higher domestic energy requirements and an increase in floor space. Homes require more energy than offices - for cooking, domestic appliances and hot water. Floor space increased by around 25% with a second bedroom built into previously unused loft space, increasing the area of potential heat loss.
  • A reduction in energy purchases for the retrofitted building as a whole of 82%.
  • With the switch in Eco Office heating from gas to electricity, carbon emissions (as calculated by standard methodologies) reduced by a smaller 77.6%. UK electricity, derived from a mix of renewables, gas, nuclear and coal, is deemed to have a carbon content of 494 grammes per kilowatt hour, high because the major fuel used is coal. Gas, ‘cleaner’ than coal, has a carbon content of 185 grammes. However, as the proportion of electricity generated by renewable sources like wind, hydro and solar increases from its current 15%, and older coal fired power stations are decommissioned, the carbon content of electricity will steadily reduce. Electricity will eventually become much cleaner than gas, and the Eco Office policy of choosing electric heating will reduce carbon emissions still further.
  • For comparison with other building developments, we have used standard methodologies to calculate carbon emissions. As it happens, however, all Eco Office and apartment electricity purchases come from a 100% renewable supplier, Good Energy, with zero emissions.
  • Electricity used by the apartment (1,425kwh purchased, plus maybe another 700 generated onsite) was more or less covered by the electricity generated from its 2.5kwp of solar tiles (1,917kwh).

The following diagram shows how these energy and carbon savings were achieved. 

Year 1 Energy Savings

Of the 82% energy purchase reduction, 46% was saved by fabric measures, (insulation, elimination of draughts and heat recovery), and 36% from onsite renewable energy generation. Of that, half came from the air source heat pump (7,415 kilowatt hours), with the rest divided between the offices solar PV (3,507 kwh), apartment solar PV (1,917 kwh), office solar thermal (947 kwh) and the apartment solar thermal (931 kwh). Residual energy purchases were 18% of pre-retrofit totals.

Incidentally, if we’d replaced the gas boiler used to heat the apartment with a second air source heat pump, or extended the office system to heat the whole building, we’d have reduced that residual energy use even further, from 18% to 11%. And because our electricity purchases are all from renewable sources, we’d have eliminated carbon emissions entirely.

Our experience has convinced us that air source heat pumps are the most suitable replacement for fossil fuels in densely populated urban areas, where the The Heat Island Effect comes into play. Surrounded by other buildings, the temperature of the air from which the Eco Office heat pump obtains its heat is consistently 2oC higher than the Met Office temperature for Liverpool. Heat pumps perform at their best when there is only a small difference in temperature between the heat source and sink, so where the environment has plenty of heat, it makes sense to recycle some of it back into our buildings.

For those with an interest in energy performance, a detailed analysis of building energy usage taking an average of the latest two years figures before retrofit provided by British Gas, compared with the first year's electricity, gas and generation meter readings is provided in the documents below.