Heat losses and gains in 5GDHC networks

Meaning of heat losses and gains

In conventional hot heat networks, the primary objective is to reduce heat losses from the water to the surrounding ground. For this reason, steel pipes are used, which are equipped with an insulation layer that reduces heat losses. The reason for the heat losses is the driving temperature difference between the hot water (e.g. 95 °C) in the pipe and the surrounding soil (e.g. 8 °C). In 5GDHC networks, the network temperature is approximately at the level of the surrounding soil. For this reason, heat losses are much less relevant in 5GDHC. In some cases, it may also make sense to keep the network temperature at a level below the ground temperature. In this case, heat inputs occur and the heat network itself acts as a heat collector, similar to an earth collector system. An interesting approach is to impose a seasonally variable grid temperature pattern: while low grid temperatures in winter allow heat gains from the ground, high grid temperatures in summer can help release excess waste heat to the ground. In this case, the ground acts to some extent as a seasonal heat storage. It should be noted that the grid temperature also affects the coefficients of performance of building heat pumps and can affect the use of waste heat sources. Heat inputs in the summer can cause the temperature in the cold pipe of the 5GDHC network to increase. This can affect the ability to provide passive cooling. For this reason, some 5GDHC systems use insulated plastic pipes.

Optimization of the network temperature profile

Determining an optimal network temperature curve for 5GDHC networks is complex. Some issues that play a role for the optimization of a temperature profile are:

• At what times of the year does heat surplus prevail in the grid?
• What is the profile of the undisturbed soil temperature over the year?
• Does the network temperature influence the efficiency of the heat generation in the energy center? If air-source heat pumps are used, for example, their coefficient of performance decreases as the network temperature rises.
• What are the temperature requirements of the buildings? While there are usually no restrictive temperature requirements for heating due to the installed heat pumps, the network temperature can be decisive for the provision of (passive) cooling. For passive air conditioning, the grid temperature should not exceed 16 °C.
• Which storages are installed for the 5GDHC network? When using ground ice storage, the nework temperatures must be lower than when using sensible heat storage.
• When using geothermal energy: What temperatures can be extracted from the ground? This depends on the properties of the geothermal field.

Theory and research

The theory of heat losses in heating networks is well researched for conventional pipe systems. A recognized set of formulas has been published by Wallentén [3]. For special applications, further approximation formulas have been presented, e.g. for the calculation of heat losses and temperature changes in double pipes (flow and return within a single plastic casing pipe) by van der Heijde et al. in [4]. In nPro, heat losses are calculated according to the set of formulas in DIN 13941. The latest research approaches rely on simulation methods rather than heuristic approximation formulas. One example is the study [5] by Dalla Rosa et al. in which an FEM model (finite element method) is used to determine the heat losses for a number of different pipe configurations.

Undisturbed ground temperature

For the estimation of heat gains and losses, the undisturbed ground temperature is an important influencing factor. The undisturbed ground temperature means here the temperature which could be measured in a far distance to the heat network and is not influenced by the heat losses. There are different calculation models to determine the ground temperature. Some very simple models are based on sinusoidal curves, which are calibrated using air temperature measurements. Other models are very complex and include a variety of physical effects in the calculation, such as humidity, wind speed, solar radiation or precipitation amounts. For the calculation of the ground temperature in the nPro tool, a model of medium complexity is used, for which only a few weather data need to be known. The model was calibrated using a large number of locations in Germany. Input data of calculation models for soil temperature are, besides weather data, the physical properties of the respective soil type. The following table lists the properties of different soil types. Further information on soil properties as well as extraction capacities of geothermal probes and collectors can be found in VDI 4640. It should be noted that the near-surface temperature in cities can be higher than in rural areas. In one study, a more than 4 Kelvin higher ground temperature was measured in the city of Berlin than in the surrounding, rural areas.

Calculation of heat losses in nPro

In the nPro tool, the heat losses and gains of a 5GDHC network can be estimated at an early planning stage. This makes it possible to determine optimal network temperature profiles and to maximize passive heat dissipation in summer and passive heat absorption in winter. The following figure shows the result of an exemplary calculation, where a typical course of the network temperature was assumed. The network temperature describes the mean value of the temperature of the warm and the cold pipe. Thus, a network temperature of 30 °C in summer with a temperature spread of 8 Kelvin means that the warm pipe is operated at 34 °C and the cold pipe at 26 °C. In the graph below, the heating (positive) and cooling (negative) loads at the energy center are plotted for 2 weeks in summer - once with heat losses (red) and once without (gray). It can be seen that the heating load at the energy center is slightly reduced due to heat losses. At the maximum, 75 kW are dissipated to the surrounding ground. Over the entire year, heat losses are 217 MWh and heat gains are 31 MWh. The net heat losses are therefore 186 MWh.

Figure 1: Illustration of an optimized network temperature profile for a 5GDHC network in the nPro tool.

Sources

1. Investigation on Relative Heat Losses and Gains of Heating and Cooling Networks. V. Madan, I. Weidlich, Environmental and Climate Technologies, 25, 1, 479-490, 2021. DOI: 10.2478/rtuect-2021-0035
2. Large-Scale Geothermal Collector Systems for 5th Generation District Heating and Cooling Networks. R. Zeh, B. Ohlsen, D. Philipp, D. Bertermann, T. Kotz, N. Jocic, V. Stockinger, Sustainability, 13, 6035, 2021. DOI: 10.3390/su13116035
3. Wallentén, P. (1991). Steady-state heat loss from insulated pipes. Byggnadsfysik LTH, Lunds Tekniska Högskola.
4. Modelling steady-state thermal behaviour of double thermal network pipes. B. van der Heijde, A. Aertgeerts, L. Helsen, International Journal of Thermal Sciences 117, 316-327, 2017. DOI: 10.1016/j.ijthermalsci.2017.03.026
5. Method for optimal design of pipes for low-energy district heating, with focus on heat losses. Rosa A. D., Li H., Energy, 36 (5), 2407–2418, 2011. DOI: 10.1016/j.energy.2011.01.024

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