What is district heating and the different types?

Introduction

District heating systems can be divided in various ways based upon their characteristics (most commonly network temperatures). There are various types of district heating with different heating sources.

Reduction of supply temperatures brings additional benefits to an overall energy system. They should be considered when developing new ULTDH and NTDH systems since these changes enable operational cost reduction and increase economic feasibility of the needed investments.

District heating can be built in different urban environments, including new areas, current areas etc.

For DH networks in transition, some authors recommend the use of 65-70°C as the optimal forward temperature for DH networks, since lower temperatures require high investment, among others DH booster HP units in each dwelling.

When estimating the potential for the integration of renewable energy sources in district heating and cooling systems, the type of system and development stage is of large significance. Here, we categorize DH systems into:

Existing conventional DH system
Existing LTDH system
Potential LTDH system in existing town
Potential LTDH system as a green field project

For existing systems, it is very important to consider what type of energy supply is already present, as it is the replacement of the existing supply that determines the feasibility of potential new supply. It is also important to distinguish between a LTDH system or conventional DH system, as this will influence the feasibility of many heat sources. In new potential LTDH systems the supply needs to be able to compete with other individual supply alternatives. In an existing town, it is important to examine if the buildings are compatible with LTDH (temperature range 50-70 °C) or need refurbishments. In green field projects, the planning of new buildings or building refurbishment and LTDH supply should be carried out in a coordinated manner. Finally, it should be mentioned that waste heat recovery and integration of renewable energy sources can also happen in ULTDH (Ultra Low Temperature District Heating) and NTDH (Neutral Temperature District Heating). ULTDH is at a temperature level (as low as 45 °C) high enough to supply space heating in new buildings, but needs heat pump boosting for domestic hot water, while NTDH is at a temperature level (in the range of 15-35 °C) that needs heat pump boosting for, both space, heating and domestic hot water.

Download the REWARDHeat database of existing next-generation DHC networks here

District heating and the different types

Conventional district heating

Conventional district heating systems are characterized by high temperatures of water in the system and use of fossil fuels as heat sources. These two characteristics cause: dependency on foreign countries that export fossil fuels, excessive emissions of CO2 and high heat losses in the thermal network. So, to reduce these negative effects, or even to eliminate them, new district systems are being developed. They are usually named as follows: low temperature (LTDH), ultra-low - (ULTDH) and neutral temperature (NTDH) district heating system. Operating temperatures of these systems are much lower than in conventional district heating systems which enables incorporation of low-temperature renewable energy and waste heat sources in the thermal network.

The main difference between mentioned district heating systems is the supply network temperatures and end-user substation type. Depending on temperature of the network, different consumer substations for space heating (SH), space cooling (SC), and domestic hot water (DHW) preparation are used. It must be noted that there is no universal definition of these systems regarding network temperatures, system configuration or consumer substations because these systems are still developing.

Supply temperature reduction improves performance of renewable heating technologies, either by direct utilization (e.g., solar thermal collectors) or by use of heat pumps. Furthermore, temperature reduction has positive influence on central heat supply units, such as co-generation and heat-only boilers. Finally, it gives possibility for decreased heat losses in distribution networks.

On the other hand, low temperature DH systems have lower temperature difference between supply and return lines. Thus, it is subject to increased investment and pump operation due to the larger volume flow rate for a fixed heat delivery. Besides this, there are additional aspects which should be considered when developing ULTDH and NTDH networks, such as additional investments needed for substation and decentralised heat booster technologies.

Advanced page: Conventional district heating (3GDH)

Low temperature district heat (LTDH)

In LTDH systems temperatures can reach up to 75°C. LTDH network supply temperatures are mostly in range between 55 and 70°C. These temperatures are high enough for space heating and theoretically for domestic hot water preparation. To enable DHW preparation with such supply temperatures, instantaneous heat exchangers and district heating storage units on primary side are being implemented at the consumer substation.

LTDH systems are sometimes considered as the fourth generation of district heating (4DH), the term coined by Henrik Lund et al [1].. The evolution of DH systems is shown in Figure 1. It can be noticed that every next generation follows supply temperature reduction and increase of energy efficiency, due to lower distribution heat losses.

Advanced page: Low temperature district heat (4GDH)

Ultra low temperature district heat (ULTDH)

ULTDH systems operate with network temperatures up to around 50°C. These temperatures are high enough to satisfy consumer space heating needs, but for the domestic hot water preparation a booster device is needed to prevent Legionella growth. Different technologies could be used for temperature boosting such as heat pumps, electrical heaters, solar collectors, boilers, etc.. In Lund et al [1] ULTDH and NTDH networks are also described as the fifth generation of district heating (5DH).

Advanced page: Ultra low temperature district heat (5GDH)

Neutral temperature district heat (NTDH)

NTDH systems have such low temperatures (up to 35°C) which are not high enough both for space heating and domestic hot water preparation. So, every consumer substation is equipped with booster heat pumps to raise temperatures to desired levels needed for SH and DHW preparation. Due to the low temperature regime, these systems also offer the possibility of space cooling. Furthermore, these networks can enable bidirectional energy exchange between supplier and customer.

NTDH networks in smart energy systems enable power-heating sector coupling through heat pumps, solar thermal and photovoltaics integration. Secondly, they can offer bi-directional energy exchange between network and the users. This could be a revolutionary aspect since the thermal price is not the main economic driver anymore. Both network operator and end-user can have benefits of such a configuration. Thirdly, NTDH can potentially have interconnection with the gas sector. For example, biomethane could be used as a back-up boiler or integrated with DHW production in a multi-purpose heat pump. In combination with thermal storage, it can offer great flexibility.

Advanced page: Neutral temperature district heat (5GDH)