Case study: Solar district heating with thermal storage
In this case study, a district with district heating and district cooling network is partially supplied by solar thermal collectors. The entire planning process can be carried out in the nPro tool: From the demand calculation, to the pipe dimensioning, to the design of the components in the energy center and the solar thermal collectors.
An energy supply for an exemplary district with 6 buildings is planned. The location of the districts is assumed to be Berlin (Germany).
In a first planning step, the demands for heating, cooling and electricity are determined for each building. It is assumed that all buildings are supplied with space heat and domestic hot water via a district heating networks. Space cooling and process cooling (e.g. server cooling) are covered by a district cooling network. Plug-load demands (e.g. for lighting) and electric demands for e-mobility are the electricity demands of the district. The demand profiles for each of the 6 buildings can be created in nPro with a few clicks. Annual profiles with hourly resolution are of great importance for the calculation of the districts, since renewable sources (here solar thermal collectors) strongly depend on the daytime and season.
|A||Hotel||20000 m²||1700 MWh||640 MWh||980 MWh||3000 MWh|
|B||Office||18000 m²||1170 MWh||144 MWh||1062 MWh||936 MWh|
|C||Retail||24500 m²||1593 MWh||74 MWh||784 MWh||2352 MWh|
|D||Museum||16500 m²||1073 MWh||83 MWh||578 MWh||859 MWh|
|E||Theater||6700 m²||503 MWh||40 MWh||101 MWh||563 MWh|
|F||Restaurant||2500 m²||188 MWh||125 MWh||50 MWh||358 MWh|
Load at energy hub
After adding all buildings to the district, the load profiles can be displayed in nPro in hourly resolution. Figures 2 to 5 show the demand profiles at the energy center for heating, cooling and electricity.
In nPro, the diameters of the pipes for district heating networks can be dimensioned semi-automatically. In this design example, the assumptions listed in Table 2 are made for the heat network calculation.
|Parameter||District heating||District cooling|
|Relative heat/cold losses||12 %||8 %|
|Pump work (share of heat/cold supply)||1 %||2 %|
|Temperature spread||20 K||10 K|
|Pipe roughness||0.1 mm||0.1 mm|
Figure 6 shows an example of the dimensioning of the supply pipe to buildings B, C and D. In the preliminary design, the nPro tool suggests three pipe diameters for this network section. In this case, these are the diameters DN150, DN200 and DN250. nPro then calculates the maximum pressure drop for each of the suggested diameters, as well as other key figures such as the annual pumping work or maximum flow velocities. Based on this information, the user can select the optimal pipe diameter.
The optimized pipe diameters for the heating (red) and district cooling network (grey) for this district are shown in Figure 7.
Design of the energy center
In this district, the heating and cooling supply is supported by solar thermal collectors. For the design, it is assumed that at the energy center a connection to an existing district heating network is available to cover base heat load. Heat from solar thermal collectors is used directly in the district and, it is possible to feed surplus heat into the district heating network. The heat generation from solar thermal collectors is shown for incident angles of 60° and 15° in Figures 8a and 8b, respectively.
Figure 9 shows the operation of the energy supply system optimized by nPro. The cooling loads are covered by compression chillers. The optimal solar thermal area is 5000 m², which is the maximum installable area for this district. The solar thermal collectors produce 442 kWh of heat per m² of collector area per year. 24.3% of the heat demands in the district are met by the solar thermal collectors, and the remaining 75.7% are met by the external district heating connection. The electricity to cover the building demands and the compression chillers is purchased directly from the power grid. Figure 10 shows the heat purchased from the external district heating network. It can be seen that in summer, almost no more heat needs to be imported from the external district heating network due to the solar thermal system and the heat storage.
The operation of the thermal storage (Figure 11) shows that the storage is charged in the afternoon hours in summer and discharged in the evening/night hours. nPro also provides an economic analysis of the energy supply system. Figure 12 shows the evolution of the net present value over the lifetime of the system. A positive net present value at the end of the project life indicates that the investment pays off. The environmental analysis in nPro shows that the CO2 emissions of this district are 4365 tons per year.
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Plan and optimize district energy systems with nPro