Time for centralised heating plant to make a comeback

Akva VX 2000
Significantly reduced carbon emissions can be achieved on apartment developments with centralised heat generators supplying interface units in each apartment. John Cooper explains.The UK Government has recently committed to cut greenhouse gas emissions in this country by 80% by the middle of this century to combat the threat of climate change. In this landmark announcement, made to Parliament on 16 October 2008, Climate Change & Energy Secretary Ed Miliband said that the current 60% target would be replaced by this higher goal. He also signalled help to encourage small-scale electricity generation though technology such as solar panels and wind turbines and measures to promote the use of renewable fuels and nuclear power. For those of us in the building services sector thinking that the credit crunch would have a detrimental effect on the specification and adoption of carbon reducing technologies, Ed Miliband was also at pains to point out that the Government would not ‘row back’ on green issues in the light of the current economic crisis Buildings account for nearly half of the UK’s CO2 emissions. Since most of these emissions are generated by the energy-consuming services used in heating, cooling and ventilating buildings, the performance of our industry remains central to the Government’s ambitions. From the strong line Mr Miliband is taking, we can be pretty sure that measures and incentives will be put in place to help achieve these targets and support the regulatory framework provided by Building Regulations, the Energy Performance of Buildings Directive (EPBD), the Code for Sustainable Homes and other legislation. In this context, apartment projects present a particular set of challenges to the building-services design team. Yet, if we take a decentralised approach to the design and installation of heating and hot water systems on this type of development, whether for a brand new scheme or a conversion, we can improve energy efficiency, reduce CO2 emissions and help future-proof the fuel supply being used. This is how it works. A central boiler plant, CHP or district-heating substation typically acts as the primary heat source and works in combination with decentralised units (also known as interface units) in each apartment to control the supply of heating and domestic hot water. These interface units incorporate one (for direct heating systems) or two (for indirect heating systems) compact plate heat exchangers which deliver instantaneous water heater to each apartment on demand and control the heating flow to the occupants’ radiators. Systems can be designed with a built-in energy meter and water meter to monitor energy and water consumption and, if required, bill for it. In applications where the central heating plant has oil- or gas-fired boilers, the primary flow temperatures can be adjusted according to the ambient temperature, ensuring high energy efficiency performance at all times. Typically, a temperature of 80°C which is required in the winter can be reduced to 60°C in summer when heating is not required. The concept of a decentralised unit in each apartment also opens up the opportunity of using combined heat and power (CHP) plant, with its well documented efficiency of heat and power generation. The decentralised approach is not just limited to large apartment complexes. In smaller systems, say less than 100 apartments, heat is transported from the heating plant to the unit in the apartment via a buffer tank, which has been designed so that stratification is obtained. The buffer tank avoids frequent starts and stops of the burner in a boiler installation, thereby further reducing energy consumption and harmful emissions. In a district-heating installation, the buffer tank creates a more constant load on the control valve of the primary heat exchanger. It also stores sufficient energy to provide an instant supply to the domestic hot water heat exchangers in each flat during peak-load periods. The buffer tank is normally sized to supply a 10 minute peak load, based on diversity. In larger systems, the piping volume will provide a sufficient buffer volume to accommodate the large load changes in the use of domestic hot water. Unlike conventional central-plant solutions, which require large volumes of central generated domestic hot water to be stored at 65°C, the use of localised interface units eliminates heat loss in the DHW distribution network and storage cylinders (because there aren’t any). The problem of legionella is also addressed because the DHW is not stored. Additional energy efficiencies can be achieved by local control of room temperatures using programmable thermostats to control the heating circuit. Meanwhile, the system also ensures that accurate energy bills can be easily created for each apartment, by enabling remote reading of the energy consumption from an accurate energy meter. This type of system is also independent of the energy supply; gas, oil, heat pump, wood, solar or district heating can all be used. This flexibility offers a considerable advantage to system designers in the current climate where there are doubts over the long-term security and price of established supplies, and certain to be more pressure to accommodate greater use of renewables in the future. Other advantages include lower capital investment costs, being able to maximise the use of space in the building for its actual purpose, and, of course, the performance of the system itself in satisfying the heating and hot-water needs of the apartment owners. Taconova’s Danflat approach, for example, is proven in numerous installations across Continental Europe and a growing number of UK projects. From a number of perspectives, it is in step with the climate-change agenda, providing a much more energy, cost and space efficient solution than conventional solutions based on electric or gas boiler and DHW cylinder. John Cooper is managing director of Taconova (UK)
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