COOL DH has demonstrated high efficient District Heating networks and solutions using a number of innovations. Each innovation is presented in the following sections under demand, distribution and supply.
Below we summarise some of the main findings so far. More information on the findings can be found on the Deliverables-section of this website:
On the demand side we find end consumers of DH.
Solutions for avoiding risk of legionella
The challenge of LTDH is the temperatures needed for production of Domestic Hot Water (DHW). Lower temperature increases the risk of legionella and with today’s standards the heat limit is above 50-55oC for circulation water. 55oC is a legislative demand in Sweden.
However, several reports discuss this topic and in some countries 45oC is now allowed for plug flow installations with small volumes, that is when substations or micro heat exchangers are used. Another method is to heat the water to 45oC and sterilise it. A study on the legislative rules for Sweden, Denmark, and Germany etc. will be conducted and made public. The study will also evaluate different solutions on how to avoid legionella (for example micro electrolysis, UV treatment, use of substations in dwellings etc.).The report and will form the basis for solutions that will lead to implementation further on in the project.
The research found that only in countries which use the exception of the three-litre-rule in small systems, decentralized substations that heat water to 45-50oC can be used together with ULTDH. Otherwise, central DHW preparation / temperature topping to 50-55oC (60oC) is required. Or electrochemical water treatment can be added to avoid bacteria growth.
Local integration of renewable energy sources
COOL DH will work on solutions for preparation of DHW by preheating with LTDH and local supplementary heating with renewable energy on a substation level.
COOL DH will also investigate and advice on the use of various substation solutions for single-family housing units. It will also find system solutions for multifamily houses and tertiary buildings like offices and institutions.
Of the many solutions identified, air-to-water or PVT heat pumps may be a good alternative to supplement the DH at the consumer. Split flow solutions with heat recovery from the ventilation system are a promising solution in multi-family houses. However, a deeper analysis of each technology is required for each specific project, where the entire installation requirements and energy consumption can be evaluated precisely.
Substation solutions: Substations with heat exchangers designed for LTDH operation (better cooling of DH water) and better insulation and controls.
System solutions for multifamily/tertiary buildings: Analysis showed that substations in the individual flats do not increase energy efficiency in multifamily buildings. They tend to lead to higher investment and operational cost compared to a traditional solution with only one substation.
Too high return temperature
The LTDH system is monitored by direct connection to the consumers heat meters. This can be used to monitor heat losses in pipes, map consumption pattern and diagnose optimisation potential etcetera. However, today’s meter may not have an optimal resolution and sampling time for analysing the system performance in detail for short periods. COOL DH will investigate what is needed for sufficient evaluation of the system and how to read and process the data to be used for return temperature optimisation.
New design concepts for optimisation of LTDH distribution systems
Traditionally, the DH-systems are oversized with a large safety margin, causing high system losses between 17% up to 35 % where energy density is low. About half of the heat loss is in the connection lines of the consumers. By hydraulic optimisation and decentral buffers the pipe dimensions and thereby the heat losses can be reduced, but the pumping energy and operation pressure of the system raise. COOL DH will use thermal and hydraulic simulation (using TERMIS and NETSIM) on actual pipes with a potential improvement of 50% reduction in heat losses. The system and its parameters will be optimised and presented to show the benefits.
Optimising network design (pipe insulation, pipe technology/sizing, network length optimisations) showed that significant reductions in heat loss can be achieved from typically 17% to about 10%.
New pipe components for LTDH distribution systems underground and new multimedia pipe types for LTDH distribution systems
The COOL DH project will innovate on the use of multimedia pipes i.e. pipes that carry different media and/or media at different temperature levels, as well as minimising pipe losses to zero or even negative. This is achieved by including a pipe that collects the heat loss and heat from the ground. The heat will then be returned to the inner pipe, using a heat pump.
Traditional pipes are made of steel for big dimensions and alu pex for smaller than diameter of 80 mm. These pipes are normally designed for 6 bar(o), 85 oC-110 oC in a 30 years lifespan. With the goal to reduce heat loss with 50%, COOL DH will introduce pipes which are not available on the market presently i.e. flexible multilayer polymer for 16 bar(o), 62 oC, 30 years.
Furthermore, COOL DH will evaluate connection of pipes with a new improved type of push coupling of pipes. The aim is to introduce push fittings for underground LTDH pipes in smaller dimensions which would save much installation time. Today this technology is only used for indoor plastic pipes for HVAC. The new pipe connection types will be tested in laboratory.
New PE-RT pipes up to dia. 116 mm single pipes 16 m for mirror welding. PE-RT twin pipes up to dia. 65 mm for press fittings. Pipes are suitable for LTDH with pressure rating up to 13 bars. Fittings for electro welding are to be approved.
New high efficient pipe types for internal distribution in buildings
Distribution losses are not limited to underground DH pipes but are also found in and between buildings or building sections with up to 15 % of the total energy need for energy renovated or new low energy houses. Pre-investigations indicates that this loss can be reduced by 50-65 % with better pipe layout and use of PUR insulated pre-fab DH pipes inside buildings. COOL DH will innovate the use of fire safe cuffs for pipe penetrations of building components and new ways of branching of pipes will be added.
DH pipes twin and single for internal mains is an energy efficient solution, but it requires fire manchets are use when passing fire partition walls and decks. With DH substations in the flats only 3 supply pipes are needed vs normally 5 pipes. This reduces heat losses from pipes.
Optimising cascade couplings for optimal use of low-temperature sources
COOL DH will study and innovate on optimal connections principles and controls of heat pumps/coolers to the LTDH system for both Lund and Høje-Taastrup.
When producing district heating based on low temperature heat sources of 7-23oC, the highest overall COPH is achieved when similar heat pumps are used in cascade coupling and are working under similar conditions. R717 (Ammonia) resulted in the highest COPH of the refrigerants investigated. Integration of additional renewables. Use of an intelligent shunt at the connection points for the LTDH system provides an ideal location for integration of low temperature heat sources from renewables or surplus heat available on the low temperature shunt line instead of adding renewable energy to the supply line at higher temperature.
Development of short time and seasonal energy storage in Høje-Taastrup
During the summer, the peak load for district cooling will benefit from having cold wells in an ATES (Aquifer Thermal Energy Storage) buffer to draw from. During warm summer periods, the need of the LTDH system will be low. Surplus heat is therefore abundant and can be stored in the warm wells of the ATES system.
When a ATES system is available, heat pumps can extract the heat when needed and supply it to the LTDH system and regenerate the cold wells for supply of district cooling. The heat pump can be controlled via price signals and/or weather forecasts/weather data, producing heat when wind/solar is abundant, allowing for a greater overall portion of renewables in the electricity grid as well. Innovative ways of using such heat pumps in will be studied in Høje-Taastrup.
If the ATES cannot be realised due to water interests or hydrogeological conditions a borehole energy storage system will be considered.
A big heat storage of 70.000 m³ (30 MW, 3300 MWh) in Høje Taastrup (Denmark) that has been investigated during the project, was approved and completed early 2021, with an investment of 10 million €. It is planned for 26 cycles per year.
Beyond the state of art – expected results
The project developed new DH pipes offering new characteristics in the following fields:
- use of PE-RT plastic material instead of PEX and steel
- improved insulation material
- integrated oxygen and vapour barrier
- weldable coupling methods
- leak detection system; higher flexibility
- larger dimension than normal PEX.
The expected impacts are as follows:
- Installation of a 1.342 MW heat pump at CITY2 shopping mall in Høje Taastrup (Denmark) in 2020-2021 with expected 3000 yearly full load hours, providing 4.03 GWh/year LTDH and 2.97 GWh/year district cooling. Primary energy savings: 4.2 GWh/year.
- Installation of 350 m of recovery pipes along a main distribution pipe that connects CITY2 to the renovated district heating network in Østerby in Høje Taastrup. A heat pump supplies delivers 80.6 MWh to the LTDH network. Primary energy savings: 38.4 MWh/year.
- Installation of a prosumer heat pump (1.92 MW heating and 1.5 MW cooling capacity) at Nordea Bank in Høje Taastrup (Denmark) in 2020 with expected 6500 yearly full load hours, providing 12.5 GWh/year in LTDH plus 11.7 GWh of cooling. Primary energy savings: 17.4 GWh/year.
- In Lund the share of renewable energy in the DH network will increase from 98% to 100%; in Høje-Taastrup from 51% to 90% for the served area in Østerby.
- The heat losses in DH systems are usually more than 17%, in the actual case more than 35%. Network optimisation based on Termis simulations will reduce these losses to 11% in Østerby and even less in Brunnshög district in Lund (Sweden. This will reduce heat loss payments by 10-25% for tenants in Østerby.
- There are more than 800 DH utilities in Sweden and Denmark alone with areas that could be converted to LTDH.