Disctric heating system components

District heating and cooling system has different components, as illustrated in the figure below. Thermal supply constitutes of the thermal source, coupled with supply technology and thermal storage, thermal network, and end-user substation.

In this section, different types of end-user substations, thermal networks and thermal sources are shown. These system components can be combined in numerous different ways. In this document, the most common combinations are shown while focusing on ultra-low temperature (ULTDH) and neutral temperature (NTDH) district heating system. Main characteristics of such networks, and comparison with traditional low-temperature (LTDH) systems are shown below.

LTDH

Temperature regimes around 55-60°C

Thermal losses relatively high (~20%)

RES and urban WH must be boosted via HP before thermal network

No need for boosting technologies in end-user substations

Simultaneous heating and cooling not possible

ULTDH

Temperature regimes around 35-45°

Thermal losses relatively low (~15%)

RES and urban WH are usually boosted via HP before thermal network

Need for DHW temperature boosting in end-user substations

Simultaneous heating and cooling not possible

NTDH

Temperature regimes around 15-25°C

Thermal losses are minimal (~10%)

RES and urban WH can be directly injected in the thermal network without booster heat pump

Need for SH and DHW temperature boosting in end-user substations

Simultaneous heating and cooling possible



End-user substations

End-user substation is component which is usually located in the building of the final user. Substation is used to connect primary (thermal network) and secondary (building) thermal circuit. End-user substations are used to cover space heating (SH), domestic hot water (DHW) and space cooling (SC) demand. The type of end-user substation is directly related to temperature regimes in the thermal network. Due to this, in this analysis three substation types are defined: LTDH, ULTDH and NTDH substations. The main characteristics of ULTDH and NTDH substations are shown in Table 7, which shows main characteristics of ULTDH and NTDH substations and comparison with “traditional“ LTDH substations. Additional details on end-user substations can be found in deliverables D4.1 - Configuration and sizing of Package Substations and D4.2 - Prefabricated skids.

LTDH



LTDH substation – instantaneous heat exchanger unit
This LTDH substation is based on instantaneous heat exchanger unit. It is highly efficient and compact heat exchanger capable of providing domestic hot water on-demand.

Strengths
 - Low-space consuming
 - Cost-effective
Weaknesses
 - DHW production on demand (no TES available)
Opportunities
 - Easily upgradeable
Threats
 - System flexibility is reduced



Second LTDH substation is based on district heating storage unit. Due to water stagnation, temperature regimes must be higher that with IHEU. This solution demands higher volume for installation of district heating storage tank. Besides thermal storage unit, heat exchangers are needed domestic hot water and space heating demand.

Strengths
 - Increased flexibility (TES available on-site)
Weaknesses
 - TES investment needed
 - Additional space is needed in the building
Opportunities
 - asily replaceable with different space-demanding solution (booster heat pump)
Threats
 - Higher temperature is needed due to stagnated water (Legionella risk)

ULTDH



This substation is based on booster heat pump combined with district heating storage unit. Thermal network supply line is used both as a heat source and a heat sink for a heat pump. Hot water is feeding the district heating storage unit and/or DHW heat exchanger. Since the temperature regime of the network is suitable for space heating demand, no boosting is needed.

Strengths
 - Single heat pump per building
 - Flexible and central DHW production
 - Reduced cost (BHP economy of scale)
Weaknesses
 - Space demanding (building substation)
 - Lower booster heat pump COP due to higher temperature in TES needed
Opportunities
 - Easily replaceable with different technology
 - Flexible production could utilise low electricity tariffs
Threats
 - Water stagnation (high temperature needed)
 - DH network reduction influences BHP COP



The substation also utilizes thermal network supply pipe as a heat source, however there is no central heat pump in the substation. On the contrary, every apartment building has its own micro booster heat pump which enables DHW production on-demand. This means there is little-to-no water stagnation in the system and temperatures can be kept relatively low (around 55°C), thus resulting in relatively high COP. Thermal network temperature regime is sufficient for space heating production, thus only heat exchanger is needed.

Strengths
 - Low temperature lift, due to low water stagnation, results in high COP
 - Low-space consumption
Weaknesses
 - Every apartment needs booster heat pump
 - Investment cost intensive
Opportunities
 - Low operational cost and low vulnerability to electricity tariffs
Threats
 - Difficult to replace



This substation is using ULTDH network only for covering space heating demand. Since thermal network temperatures are sufficient, only heat exchanger is needed. However, for domestic hot water production additional locally installed technology should be used, such as ais-source heat pump.

Strengths
 - Simpler system planning
 - Utilisation of well-established technologies
Weaknesses
 - Thermal network is not utilised as a heat source (lower system COP)
 - DH connection and additional supply technology is needed (high investment)
Opportunities
 - Booster technology could also serve as a back-up technology if DH network is under retrofit e.g
Threats
 - Operational cost depends on the booster heat source (on the other hand DH temperature regimes are guaranteed)



Like the previous one, this substation is also additional booster technology to cover domestic hot water demand. In this case, the booster technology is a boiler which can use electrical energy, natural gas or biomass.

Strengths
 - Simpler system planning
 - Utilisation of well-established technologies
 - Local energy tariffs could be utilised (e.g. if biomass is locally available)
Weaknesses
 - Thermal network is not utilised as a heat source (lower system COP)
 - DH connection and additional supply technology is needed (high investment)
Opportunities
 - Booster technology could also serve as a back-up technology if DH network is under retrofit e.g.
 - Easily replaceable with other technology
Threats
 - Operational costs depend on the energy tariffs of the used fuel (e.g. natural gas)

NTDH



In this NTDH substation, booster heat pump is using thermal network warm pipe as heat source to provide space heating and domestic hot water. Two buffer tanks are used to increase flexibility of the system and secure temperature stability. Furthermore, space cooling is directly covered by using cold pipe of the NTDH network. It is important to mention that temperature regimes of the cold pipe should be around 10°C to be suitable for direct cooling.

Strengths
 - Single BHP for DHW and SH
 - Space cooling directly
Weaknesses
 - Low temperature of return needed for direct space cooling
 - Return temperature must be low enough to cover direct cooling
Opportunities
 - Simultaneous heating and cooling is possible (however rarely used)
 - Could be upgraded from existing ULTDH
Threats
 - Booster heat pump COP could be reduced due to network temperature constraint



This substation is also based on the booster heat pump. Heat pump is used to cover both space heating and domestic hot water demand. However, heat source (evaporator side) changes depending on the heating and cooling mode of the substation. In the heating mode (when space heating is needed), heat source is warm pipe of the thermal network. In the cooling mode (when space cooling is needed), heat source is return flow from the cooling system. This means that booster heat pump is used both for cooling and DHW production simultaneously.

Strengths
 - Heat recycling on-site
 - Building-level system COP relatively high
 - Only one BHP needed
Weaknesses
 - Space cooling possible only with DHW production
 - DHW BHP performance reduced since heat source is on lower temperature than network
Opportunities
 - Space cooling system could be easily upgraded with additional BHP if needed
Threats
 - DHW and SC demand should be simultaneous and balanced



This NTDH substation on reversible heat pump which provides space heating and domestic hot water during heating mode and space cooling during cooling mode. In this configuration, it is not possible to cover domestic hot water and space cooling simultaneously. During heating mode warm pipe is used both as heat sink and heat source to provide SH and DHW. During cooling mode network is used as heat sink, increasing its temperature from cold to warm pipe temperature. Heat source is return from space cooling system.

Strengths
 - High COP both for heating and cooling - Network serves as heat sink and heat source
Weaknesses
 - Not possible cover heating and cooling simultaneously - Reversible heat pump needed
Opportunities
 - TES could be added in order to increase flexibility
Threats
 - DHW operation is needed throughout the whole year, this could represent an issue



This substation includes two booster heat pumps, thus separating heating and cooling demands. One booster heat pump utilizes warm pipe for heating purposes, while the second one uses cold pipe of the network for cooling demand.

Strengths
 - The most flexible NTDH substation
 - High COP for both BHPs
Weaknesses
 - High investment cost
 - Additional space requirements
Opportunities
 - Suitable for large facilities with different simultaneous demands
 - High level of bidirectionality in the network
Threats
 - No on-site recuperation
 - High O&M costs (2 BHPs)