Solar System piping arrangements

Large solar systems serving multi-buildings typically have several heat transfer loops. First, there is one with the fluid flowing through the solar collector (solar primary loop). This one may contain an anti-freeze solution. Second, there is one that circulates from the heat exchanger with the collector fluid to the thermal storage tank (solar secondary loop). Third, may be a circulating fluid that heat the domestic hot water. Finally, there may be a circulating system going to a district or building heating system. Normally circulating systems use water as their heat transfer fluid. The types of piping systems can be characterized as:

• Solar supported local heating system with a two-pipe network:
  
  • two-Pipe Networks with Decentralized Domestic Hot Water Storages
  • two-Pipe Networks with Decentralized Heat Transfer Units
• Solar supported local heating system with a four-pipe network:
  
  • four-pipe networks with centralized energy storage and centralized domestic hot water storage
  • four-pipe networks with centralized energy storage and decentralized domestic hot water storage
• Solar supported district heating network with direct interconnection of a centralized solar thermal system:
  
  • solar thermal plant directly feeding the supply line of an existing district heating network
  • solar thermal plant directly preheating the return line of an existing district heating network.

Two pipe networks

In two-pipe networks, the heat supply to the heat sinks (buildings), including both domestic hot water and space heating, is by means of a pair of pipes. Domestic hot water is heated in a decentralized manner for the individual consumers using continuous flow water heaters, or by means of decentralized DHW storages using the charge-store principle. The two-pipe network shown in Figure 3.21 consists of centralized energy storage and decentralized heat transfer units for each building or unit connected to the network.

The energy storage is the central point for all heat flows and acts as a hydraulic gateway. To guarantee a reliable supply of heat with this design, it is essential that adequate reserves are permanently stored in the upper region of the energy storage (stand-by volume) so as to cover peak demand.

Domestic hot water is heated in a decentralized manner using continuous flow water heaters (usually via plate heat exchangers – no additional storage needed).

Two-pipe networks in combination with decentralized heat transfer units are ideally suited for use in compact unit blocks (medium to high energy densities). For less energy dense clusters of buildings (lower heating requirements per length of heat distribution pipe), a two-pipe network in combination with decentralized daily storages is preferable (Figure 3.22).

Solar-supported heating networks in combination with decentralized heat transfer units are also highly suitable for use in existing buildings. This includes buildings that are equipped with central space heating, but that also have a decentralized supply of domestic hot water (off-peak energy storage units). Whenever these energy storage units have to be renewed, they could then be replaced by decentralized heat transfer units. At the same time, improvements to the heat insulation (insulation of the building envelope, new windows) will mean that the space heating system can operate at lower temperatures.

Figure 3.23 shows a modified two-pipe network with decentralized energy storage for each building connected and additional decentralized solar thermal systems. The single consumers within the building are supplied via decentralized heat transfer units.

This concept is recommended for local heating grids with high energy densities (high heating requirements of buildings per length of heat distribution pipe), especially for new-built facilities that are constructed in several construction stages (modular enlargement).

Four-pipe networks with centralized energy storage and centralized domestic hot water storage

In four-pipe networks, the heat is distributed through four pipes. In addition to flow and return lines for the space heating system, four-pipe networks also have two pipes for the supply of domestic hot water (distribution pipe for domestic hot water and circulation line).

The four-pipe network shown in Figure 3.24 consists of centralized energy storage and centralized domestic hot water storage. The energy storage is the central point for all heat flows and acts as a hydraulic gateway. Domestic hot water is heated in a centralized manner using the charge-store principle.

Due to the high distribution losses in the DHW supply and circulation lines, this concept is especially recommended for local heating grids with high energy densities (high heating requirements per length of heat distribution pipe) such as Army New-Recruit Troop Training Sites.

The four-pipe network shown in (Figure 3.25) consists of centralized energy storage and decentralized domestic hot water storages in the buildings.

The energy supply for space heating and domestic hot water is performed by two pairs of distribution pipes using the heating water as the heat transfer medium within the whole heating network.

Due to its higher specific investment costs, this concept is especially recommended for local heating grids with high energy densities (high heating requirements per length of heat distribution pipe) such as Army New-Recruit Troop Training Sites.

The direct interconnection of a solar thermal system to an existing district heating network (Figure 3.26) is applied to provide some base load energy directly to the grid. In general, two different applications are most commonly used:

• solar thermal system directly preheating the return line of an existing district heating network
• solar thermal system directly feeding the supply line of an existing district heating network.

Applications of this kind are commonly designed based on the available space and the existing dimensions of the district heating branch on site, and not on the actual load in a specific building. The solar collectors can either be roof- or ground-mounted. The majority of these systems can be operated without additional storage as they use the district heating network as storage (as long as they provide a small amount of heat in comparison to the total load in the district heating system).