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Ground Tubes
1 Heat distribution via underground pipes – district heating in the neighborhood?
The power plants that will be operated with renewable energy sources in the future, as well as heating systems in residential areas that may work with combined heat and power (CHP), will be supplied with surplus energy from the summer half-year into the winter half-year. These summer surpluses of electrical energy must therefore be stored temporarily, for example by converting PV and wind power into liquid or gaseous energy sources using electrolysis etc. ('power-to-gas'). These can then be used in the winter months in the same way as the fossil fuels natural gas and crude oil. The waste heat generated during combustion in CHP plants or fuel cells can generally already be used very well today to heat buildings and provide domestic hot water. The energy content can therefore be used almost completely: electrical energy from the generator for light, power and communication and the waste heat for heating. This will become all the more important in the future because the available budget for renewable energy sources is likely to be more limited and primary energy will therefore be significantly more expensive than it is today, especially in winter.
The at least partial supply of individual neighborhoods via district or local heating networks is therefore an important component of the so called heat production transformation and is therefore an important option for heating and domestic hot water preparation.
Local heating networks are also interesting for the use of heat pumps (HPs) because the heat from larger central HP systems can be distributed to surrounding buildings via a local heating network. In this context, however, a warning should be issued about the heat distribution losses that then occur in the local heating networks: the prerequisite for the use and distribution of heat via heat-conducting pipes laid in the ground is that the heat distribution networks are optimized the best possible. This means that all heat distribution losses must be consistently balanced and minimized as far as possible.
The heat losses occurring by the distribution of waste heat from combined heat and power plants (CHP) in so-called local and district heating networks are considerable compared to the final energy made available to the user at the house connections. It is therefore instructive to look at the amount of energy lost per year compared to the amount of final energy supplied for different building types.
For passive houses and buildings renovated to the EnerPHit standard in particular, the 'energy density' achievable in the grid is relatively low because the final energy requirement for space heating and domestic hot water is only around 50 kWh/m²a and possibly even less, i.e. only around 6 000 kWh/a of final energy is delivered per residential unit (120 m²). For typical old buildings with a consumption of around 240 kWh/m²a or 29 000 kWh/a, the turnover is significantly higher [AKKP 46 FW]. These figures already include the heat distribution losses within the building.
In [AKKP 46 FW], various scenarios for the configuration of district heating and local heating distribution networks were drawn up and compared. Several different qualities of thermal insulation of the heat distribution pipes were compared with each other and the length of the distribution pipes in a network was varied. The results show that with a moderate improvement in the quality of the pipe insulation and an optimization (shortening) of the network length, acceptable heat distribution losses can already be achieved with today's pipe technology. So passive houses and districts renovated to the EnerPHit standard can be supplied with heat economically. The 'specific' network length in meters/residential unit (m/unit) is an important factor here. The first goal of network optimization must therefore be to keep the specific network length low and thus the network connection density high; values below 6 m/unit should be aimed for [AKKP 46 FW].
A corresponding spreadsheet is provided as an Excel tool for this purpose, which makes the heat distribution losses and their effect on the regenerative supply visible. In the tool all the geometry of the grid i.e. length of underground tubes and quality of thermal insulation can be defined. The characteristic values determined in this way can also be used in the PHPP to ultimately estimate the primary energy input for the heat supply.
