Anergy networks: why cold district heating makes neighbourhoods more efficient

01The principle: a network with almost no temperature

Conventional district heating transports water at 70 to 120 °C and loses 10 to 20 per cent of its energy to the ground along the way. An anergy network reverses the principle: it carries water close to ground temperature, between about 8 and 20 °C. Because there is hardly any temperature difference to the surrounding soil, losses approach zero – uninsulated plastic pipes are often sufficient.

Space heating and domestic hot water are generated locally by a heat pump in each building. Because its source is mild and frost-free all year round, it operates considerably more efficiently than an air-to-water heat pump in winter.

02The lever: heating and cooling at the same time

The network consists of a warmer and a colder pipe. A residential building draws heat from the warm side; an office building or data centre that needs cooling feeds its waste heat in. The direction of flow changes according to demand. The more heating and cooling demand occur simultaneously, the less the actual source – usually a borehole field – has to contribute.

Mixed-use new-build neighbourhoods are therefore ideal: housing next to commercial premises, laboratories or a data centre. It becomes more difficult in purely residential neighbourhoods and in existing buildings with radiators that require high flow temperatures.

03A neighbourhood in figures

A fictitious calculation example: assume that a mixed neighbourhood requires 2000 MWh of heat and 600 MWh of cooling per year, and that the heat pumps in the buildings achieve a seasonal performance factor (SPF) of 4.5.

−33 %less electricity for the same heat than with air-to-water heat pumps

The 600 MWh of waste heat from cooling is directly available for heating. Net, the borehole field only has to supply 956 MWh. Compared with air-to-water heat pumps, the electricity demand for heat falls from 667 to 444 MWh.

04The limit: the borehole balance

If, over the years, the neighbourhood extracts more heat from the ground than it returns in summer, the borehole field slowly cools down. The source temperature falls and the heat pumps work less efficiently – in extreme cases, the source is no longer sufficient. A balanced annual heat budget therefore belongs in every feasibility study: through summer cooling, solar heat or waste heat, for example from a data centre.

Care is also needed with domestic hot water: for 55 °C, the efficiency of the heat pump falls significantly. A dedicated strategy with a separate stage or a fresh water station is part of a good design. And because thermal networks remain in operation for 50 to 80 years, decisions on temperature and pipe cross-section have an impact for generations.

Takeaway

The anergy network shifts temperature generation into the building. Its efficiency lever is the simultaneity of heating and cooling – its Achilles’ heel is the balanced annual budget of the boreholes.

SEDC AG · Article from our internal training series “Technik Praxis”, abridged for clients and users.