70/40 – The key to low-cost energy
Simply put, heat networks are used to deliver low cost heated water to the taps and radiators within housing developments. The lower the temperature of the circuits, the lower the unit cost of heat. At Harvesters Way, 70˚C is the temperature of plant room supply to each dwelling, whereas 40˚C is that of the return, both of which can be regarded as low by industry standards. The result is a remarkably modest heat tariff of 3.7 p/kWh.
From an initial industry-typical tariff of 7p/kWh, the specified equipment at Harvesters Way has been optimised and now delivers heat energy at the much reduced tariff of 3.7p/kWh.
M & E design was entrusted to the Keenan Consultancy, Edinburgh. Working in close conjunction with SAV Systems, a lower design temperature was chosen for the primary circuit, based on 70˚C flow / 40˚C return.
This gave immediate benefits by reductions to pipe sizing, pumping power and anticipated heat losses.
Key objectives for a heat network:
• Running the system at the lowest possible temperatures
• Giving priority to the lowest cost heat generators
• Managing the mismatch between heat generation and demand
• Accurate metering of individual dwelling usage
Several combinations of heat generator were assessed for the plant room. To secure 20% carbon reductions, a pair of SAV LoadTracker CHP units were selected, supported by 3 x 1,500 ltr vessels for thermal storage. Ancillary heating provision was made using condensing and back-up boilers. The LoadTracker units at Harvesters Way keep in step with site heat demand, with surplus electricity generation being exported to the grid. Thermal storage is controlled by a self-learning system, with the objective of maximising CHP operating hours regardless of differences between heat production and demand.
Each dwelling is provided with a SAV Danfoss 5 Series Direct FlatStation heat interface unit (HIU). These supply domestic hot water though an exchanger, with primary connections made directly for radiator heating. FlatStation units keeps accurate control of temperature at the tap, delivered with short waiting times.
Every HIU has a built-in ultrasonic energy meter, contributing to the commissioning process and providing raw data for billing. Not only are such meters maintenance free, but they also provide a permanent watch for system faults.
The key to the success of this Heat Network is proper hydraulic control, from the plant room through to every single radiator. The lower the return temperature, the more energy can be held in thermal storage. At low returns, condensing boilers can then condense and CHP units can thrive on the large temperature differentials. The resulting lower gas consumption feeds directly into lower heating bills. Low return temperatures are made possible by control valves within the HIU and at the radiator outlets.
Danfoss FlatStations maintain the design temperature differential of 30˚ between flow and return. Rigorous care must be taken to avoid uncontrolled bypasses, as these lead to unwanted temperature increase of the return. Schematics are closely checked to eradicate unintentional bypass pathways. Modest savings result from displacement of grid supply by site-generated electricity to meet plant room requirements.
Good hydraulic control continues to deliver consistently low return temperatures throughout the year. The user HIUs have proved to be reliable, low maintenance and user-friendly, as well as enabling a welcome deflation of energy prices. And this can be crucial to those households facing fuel poverty.