Delivering renewable heat to commercial buildings
Heat pumps are one of the most effective ways of deliver renewable energy to commercial building, and Mitsubishi Electric has advanced their capabilities.
It’s a couple of years since air- and ground-source heat pumps were officially recognised as sources of renewable energy by the EU Directive on the Promotion of Renewable Energy Sources. Many in the industry thought that recognition was late coming, especially since heat pumps can be used virtually everywhere and at any time, unlike other forms of renewable energy.
Since then interest in heat pumps in the UK has grown, but a recent report from the Energy Saving Trust revealed a number of under-performing installations because of poor practice and application. That report did demonstrate, however, that air-source heat pumps (ASHP) could be as efficient as ground-source heat pumps and achieve seasonal COPs that are high enough for them to be classed as supplying renewable heat.
The secret is in the equipment and how it is installed. Reverse-cycle air-conditioning units perform poorly at lower ambient temperatures, with both heat output and COP falling drastically.
So what should an air-source heat pump for commercial applications look like? Mitsubishi Electric’s answer is an Ecodan unit specifically designed and developed for commercial applications. James Timbs-Harrison, who is responsible for product marketing and development, stresses that these units are specifically designed for heating and would be useless for cooling.
A monobloc unit with an output of 43 kW at an ambient of -5°C is divided into two sections, with separate refrigerant circuits so that heat can be delivered while one half is going through the all-important 5 to 10 minute defrosting cycle. Up to 16 units can be connected to deliver 688 kW of heat. The refrigerant is R407C.
It is the performance of these heat pumps at temperatures down to well below freezing point that marks them out for space-heating applications. The COP of 4.13 at a flow temperature of 35°C and 7°C ambient independently tested by BSRIA to EN14511 is good. Seasonal COPs are in the range 3.1 to 3.7 — comfortably in excess of the 2.8 required for heat pumps to be regarded as a supplier of renewable energy.
To put that COP into context, an electrical input of 1 kW extracting 3 kW of heat from the ambient air to deliver 4 kW of heat is regarded as 75% renewable heat output.
According to Philip Ord, product marketing manager for the company’s commercial heating systems division, explains that the capacity of a standard ASHP typically falls off by 20% as the outdoor temperature falls from 7°C to -5°C, with defrosting potentially reducing output by another 10%. In contrast, the commercial Ecodan unit is said to lose only 4% of capacity, including defrost.
One of the key technologies to reduce the fall in output from these heat pumps is to simulate 2-stage compression using just one compressor.
While that COP of 4.13 was with a flow temperature of 35°C, the use of R407C enables water to be delivered at up to 70°C if required, but with a much lower COP of 1.76.
Philip Ord explains, ‘These systems can deliver renewable heating effectively to a building at 40 to 50°C and only need to achieve 70°C for DHW.’
Capable as these heat pumps are, they cannot deliver their potential unless they are operated properly. Philip Ord stresses that the application of ASHPs is nothing short of crucial. Not acceptable are flow temperatures above 50°C, or the seasonal COP will be so low that no renewable heat will be delivered. Nor are traditional controls appropriate, for they will also lead to the seasonal COP being less than the 2.8 that marks a heat pump as delivering renewable heat.
One of the key elements in controlling heating system served by ASHPs is weather compensation, with the flow temperature to the heating system being reduced as the outside temperature rises. The graphs show the effect of weather compensation on achieving the best COP, especially when the outside air temperature rises above 5°C.
And as Philip Ord explains, the way a building is operated improves the effectiveness of heat pumps. He says that it is best to run a heat pump at part load and maintain the temperature in a building than to reheat quickly from cold (say 15°C) to a comfort temperature of 21°C using the full output of the heat pump. He suggests heating the building 24 hours a day, allowing the temperature to fall back to 19°C when it is not occupied.
Another important aspect of maximising efficiency is how many units are operated to meet the demand for heat. The compressors have variable-speed drives using frequency inverters, and their COP falls off at higher and lower frequencies. So rather than having two modules in a multiple installation operating at less than optimum capacity because the compressor speed is too slow, one unit is switched off. Likewise if a module is operating at less than optimum capacity because the compressor speed is too fast, another module is started. The control system also rotates the operation of modules based on compressor run hours.
What of the potential for air-source heat pumps in commercial-scale developments. Jodi Willis of consultants MTT is an enthusiast based on experience with a number of large apartment developments. They include a high-end residential development of 530 apartments on Kensington High Street in London to provide space heating. Another development of accommodation for 300 students in Kensington uses air-source heat pumps to preheat DHW.
Jodi Willis points out that air-source heat pumps are classified by BREEAM as an LZC technology and also an approved technology by the Greater London Authority. He stresses that the GLA accepts air-source heat pumps as a renewable technology and very good value for money in buildings with high heat loads.
The effectiveness of heat pumps at delivering renewable energy and reducing carbon emissions will improve as the carbon factor of the grid falls from its present 0.515 kg of CO2 per kWh of electricity to 0.322 kg/kWh by 2025, according to the Department of Energy & Climate Change. But even now, savings are possible, and will get better.
