Energy supplier saves significant energy: replace not refurbish decision reduces costs, consumption and carbon emissions
When EDF Energy needed to replace the chillers in its main HQ office buildings in Barnwood, Gloucestershire, in early 2015, R&S advised them against taking their planned course of action…
The offices had been serviced by two Dunham Bush water-cooled heat recovery chillers, installed over 40 years prior. Only one of these had been operational during the previous few months and reliability was a major concern.
The buildings were heated by a combination of chiller heat recovery and off-peak direct electricity with heat stores. The original refrigerant was R22 but had been changed to operate on RS-45 (R434A). The conversion was successful but the machines only had a single refrigeration circuit with a refrigerant charge in excess of 1,000kg. Some energy was drawn "off-peak".
Both machines were going to require a major refurbishment for the future operation of the air conditioning system. EDF Energy proposed to change both units on a near like-for-like basis over a two-year period, which they put out to tender to a number of prospective contractors.
On receiving a request to tender R&S carried out an analysis, comparing the options available. We examined the advantages of using modern heat pump technology to take on both heating and cooling of the office block, rather than replacement or repair. Other factors we took into consideration were:
- a new residential housing development had been built 50 m downwind of the evaporative cooling towers;
- the original off-peak electricity cost for the direct electric heating has risen from approximately 20% of peak rate to 66% of peak rate: off-peak is considerably less financially attractive now;
- the original Dunham Bush machines operated with 3.3 kV motors, whereas modern machines of the capacity required for this project operate on 400V.
We faced some significant project challenges, including:
- 1,000 kW of chilled water capacity was required by 1 May 2015;
- the entire mechanical installation and associated electrical distribution had to be completed in seven working weeks with only a two-week lead-in for mobilisation;
- the work had to be completed on a limited budget of only £400,000 – and without affecting the operation of the single Dunham Bush chiller should it be required.
By demonstrating significant cost savings by changing the original specifications we were able to persuade our client to change direction and install modern, air-source heat pumps rather than choose like-for-like replacements.
Our solution comprises:
- three Carrier 500 kW R410A air-source heat pump chillers located on the external plant area at low level, with the ability to add a further chiller for future resilience;
- rationalisation of existing 400V / 415V LV power to release three 400V supplied for the new pumps;
- retention of existing single operational Dunham Bush chiller while we commissioned the new plant;
- the buildings are cooled by operating the air source heat pump chillers in "cooling mode" with a chilled water supply temperature of 6°C at an energy efficiency ratio of approximately 2.8;
- pre-heat is supplied by operating the air-source heat pumps in "heating mode" with a water supply temperature of 35°C and a coefficient of performance at approximately 2.5;
- "stage 1" heat recovery with refrigerant de-super-heaters at approximately 50°C when the machines are operating in a "cooling" or "heating mode";
- a facility for a future ("stage 2") heat recovery to further reduce energy consumption and carbon emissions when investment funds are available.
The benefits of applying air source heat pump chillers with "stage 1" heat recovery include:
- chilled water is available with improved resilience and a lower total refrigerant inventory;
- the combined cost of electricity for heating and cooling will be reduced by approximately £25,000 each year;
annual carbon emissions will be reduced by approximately 95 tonnes of CO
The option for the future investment in "stage 2" heat recovery would yield a further annual energy cost reduction in the region of £10,000 and 50 tonnes of CO
In addition, the elevated Legionella risk associated with the cooling towers and new housing development has been eliminated and the cooling towers decommissioned.
"The total cost of ownership over a 25-year period has the potential to save over £1 million and 2,500 tonnes of CO
2 ," comments Phil Matthews.