Harness the power of your air conditioning

In these days of rising energy bills, what is the most cost effective way of heating your building? What are the alternatives to traditional carbon-based systems and is it possible to maximise efficiency by using the excess heat generated by your air conditioning?

With the UK’s buildings accounting for almost half of all the country’s CO2 emissions – far more than transport or industry, our demand for heat in the built environment is the single biggest reason we use energy in our society.

This demand is more than that required for the generation of electricity, yet switching just 12% of the requirement for heating away from fossil fuels to renewable technologies would help the country to meet around a third of the tough renewable targets we are legally bound to meet. It would also help reduce our reliance on fossil fuels and could assist hard pressed businesses trying to reduce their monthly outgoings.

Air conditioning is now used in an increasing number of commercial properties, especially as the amount of heat-producing computers and other equipment rises year on year. Modern, inverter-driven VRF systems have also advanced significantly from the early ‘cooling’ days of the industry and now offer simultaneous heating and cooling to offset demand in one part of a building with the requirement in another.

Developments over the past three or four years have provided a real opportunity for building operators to both lower emissions and heating bills with additions to the range of air conditioning products that can generate all of the hot water a building needs.

This means that anyone using an air conditioning system now has the opportunity to transfer heat energy into their building’s water supply to remove the need for a separate boiler system.

Opportunities available

When communications company Garnett Keeler moved into its premises in Carshalton, Surrey, it inherited a complete mishmash of building services with 17 different air conditioning units and an old gas boiler.

The air conditioning at Inver House was a real mixture of brands and included R22 models which were long past their useful working life.  With the looming ban on R22 refrigerant, the company faced having to upgrade much of its air conditioning as well.

The 8,500sq ft offices had radiators around all three floors and the heating was often working at the same time as the air conditioning.  This meant that the company was sometimes paying to both heat and cool space at the same time.

“Like any business we always watch our costs and we found it completely illogical to have two systems that often seemed to be competing with each other – with both costing us money,” explained Director, James Keeler.   “When the annual service suggested we needed to replace the old gas boiler, it gave us the opportunity to examine the whole system.”

With a possible £20k bill for a new gas boiler, the company took the opportunity to examine its overall requirements and decided to replace all of the air conditioning at the same time, thereby keeping disruption to the business to a minimum.

An examination of the different possibilities also highlighted the potential for obtaining hot water from the air conditioning which allowed the company to remove both the gas boiler and the bulk of the radiator system.

Central to the new air conditioning system is a Mitsubishi Electric City Multi PUHY-EP450-YSJM-A outdoor unit which provides heating and cooling across the three floors of the building through a mixture of PLFY ceiling cassettes and PKFY wall-mounted units.

A PWFY-P100VM-E-BU heat pump boiler sits in the company’s plant room on the second floor and transfers heat from the VRF system to the water supply to provide hot water for the kitchen and rest rooms, and heat radiators in the halls.

“We haven’t yet been able to compare year-long energy bills, but we’ve even gone so far as to have our gas supply disconnected,” says Keeler.

How the technology works

The PWFY delivers water heated to 70ºC and has been designed to work with off-the-shelf water cylinders to provide customers with added functionality, ease and convenience.

It operates using a unique cascade refrigerant circuit.  This works using two different refrigerants (R410a and R134a) and two different refrigerant circuits, to maximise heat recovery from the VRF system and take water temperatures to the levels required.  The unit also has two plate heat exchangers to offer water flow rates of 0.6 – 2.15m³/h.

Linking the PWFY to an outdoor sensor means it can monitor and detect ambient conditions allowing the unit to run at times when heat recovery in a building is maximised, for example, at 15ºC and above most buildings will start to require some level of cooling, so the rejected excess heat energy can be recovered and diverted to provide a hot water supply.

Heating a building’s hot water supply during the night using the PWFY allows for further running cost reductions as the system benefits from off-peak electricity tariffs.  As the full tank of hot water is used during the day the PWFY only needs to run at partial load to top up the tank, increasing the system’s efficiency even further.   

Significant reductions

Using excess heat recovered from the air conditioning allows the PWFY to heat a building far more cost effectively and produce far less CO2 emissions than traditional carbon-based systems.

Trials of the PWFY at the manufacturer’s own headquarters building in Hatfield in 2009 demonstrated savings of 78% in both running costs and emissions over the gas system. The PWFY water heating system has been used to supply all of the hot water required by the building’s busy kitchen for the past four years.

The trial data shows that the unit costs less than a quarter of the running costs of the previous gas boiler system and about half as much as a modern gas condensing boiler system.

Mitsubishi Electric’s headquarters building houses many different divisions within the company and accommodates approximately 300 members of staff.  The office has an on-site restaurant that can seat 100 people and serves cooked meals five days a week as well as breakfast every morning.

The restaurant is very popular with employees so the kitchen staff are constantly busy preparing and serving meals from early morning until mid-afternoon. The kitchen area therefore needs constant cooling during these times and a good supply of hot water for all of the washing up and cooking needs.

The restaurant previously used an air conditioning system for cooling and a commercial gas boiler to provide hot water.

The trials showed that the PWFY and the new air conditioning system running alongside it had annual running costs of around £517, producing 2,222kg of CO2.   The previous system is estimated to have cost £2,367 to run on an annual basis at 2009 prices and produced 10,176kg of CO2 in a year.

When compared to a modern gas boiler for water heating and a split system air conditioner for cooling (with a COP of 3.5) it was calculated that the annual running cost of the total system would be approximately £1,114 and would produce 5,020kg of CO2

Using the PWFY system instead of the modern equivalent leads to a 54% reduction in running costs per year as well as a 56% reduction in the amount of CO2 produced annually.

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