Improvement of operational temperature differences in district heating systems
Description of the Project
The project deals with refinement and verification of a method to detect malfunctioning consumer stations and other sources of high return temperature in the net. The project will be carried out in co-operation between the three proposing partners where this type of problem has been found to be of importance for the operation of district heating networks.
The aim of this project is therefore to propose and verify a method for detection of the most critical consumers of the net and to develop an action programme for suitable measures to be taken for increasing the temperature difference in the respective net. The method is based on five years' experience of work with consumer station improve-ments in Swedish district heating networks. The method may be applied by DH com-panies themselves or by consulting and service companies devoted to cooling ability improvement services.
The economical gain of increasing the cooling ability is judged enormous. Only for Sweden, the annual gain of an improvement of the cooling ability by 5°C compared to today's "business as usual" is estimated to 50 - 60 Millions US$ per year.
The efficient use of district heating systems is characterised by a large energy transport capacity in relation to the pipe dimensions. In other words, the energy transport capacity of the pipes in hydraulic networks is given by the product of temperature difference of supply and return temperature multiplied by the nominal flow rate for a given pipe dimension. Therefore, large temperature differences are prerequisites for transporting the large amounts of energy of high load times. Increased cooling ability of consumer systems also allows connection of additional customers to the net when the designed connected load has been reached in practice. In addition, at low and medium load times, a high temperature difference is desirable because it can save pumping power and in many cases can reduce distribution heat losses. Low return temperatures improve also operating conditions and efficiency of CHP, heat pumps and flue gas coolers in some countries used in heat generation systems.
In practice, the achievement of large temperature differences is counteracted by mal-functions of consumer stations and their components and often also by short circuits in the distribution network, leading to higher return temperatures and in general also to increased supply temperatures compared to what would be desirable. Additionally, system designers have very often built-in over-dimensioned "reserve capacities" leading to non-optimal functions of control instruments. The results of such malfunctions are higher pumping losses, higher return and supply temperatures and therefore higher heat losses from the net. In the worst case, the net can be choked at the high load times and will not be able to deliver the necessary design power.
The reason for low cooling ability of customer systems (high return temperatures) in district heating networks can be seen in:
- Inadequate secondary system solutions for tap water and industrial water heaters
- Malfunctioning heating and ventilation systems due to component failures or insufficient dimensioning
- Component failures (control valves, heat exchangers)
- Improper set points for control equipment
- Undetected short circuits or malfunctions in bypasses between supply and return pipe.
Summary of the final report of the project
Project Goal The aim of the project is to propose and verify methods to detect the most critical consumers in the net with respect to insufficient cooling of the DH water and to develop an action programme for suitable measures to be taken to increase the temperature difference in district heating networks.
Hence, the main result of this cooperation is a methodology and the demonstration of its practical application for detecting substations that seriously affect the return temperature of the whole net in a negative way. Contributers to the project were Sweden (ZW Energiteknik AB and the Södertörn District Heating Company), USA (Fraunhofer CEE), also based on the experience of their German mother institute UMSICHT in Oberhausen and Korea (KDHC) .
The consortium worked according to the following tasks:
- Analysis of measurement data according to the Excess Flow - or Target Temperature methods in order to determine the status of the substations in the net and to identify the reasons for potentially high return temperatures.
- Identification and repair of malfunctioning substations.
- Verification of improvements based on repeated analysis of measurement data after renovation of malfunctioning substations.
- Substantiation of the importance of this work by illustrating it with examples of the economic benefits derived from improvements.
Analysis of Excess flows
The work for detecting malfunctioning substations is principally based on the evaluation of the excess flow of substations. Excess flow is defined as the difference between the actual flow of district heating water, which passes a substation in order to transfer the desired load to the customer, and the flow through a substation that works better or ideally. For this evaluation, two methods have been developed and applied: Excess Flow Method and Target Temperature Method , respectively.
Excess flow Method
This method answers essentially the question of how a substation would perform, if the return temperature were below the actual average value of the return temperature, let's say by 5°C. In this hypothetical case, each substation is assumed to contribute to the same lower average return temperature, i.e. Reference Return Temperature (RRT) according to its own heating capacity. Lower return temperature means lower mass flows. Hence, this hypothetical approach towards a new average temperature will indicate a possible excess flow, telling us, how much each substation affects the actual return temperature of the district heating network.
Target Temperature Method There is, however, a practical limit for the achievable return temperature in a substation. Such limiting factors are design return temperatures of the house heating system, the temperature of the incoming cold city water and the requirements of the hot water circulation as well as the physical conditions of the heat exchanger. Therefore, an ideal Target Return Temperature (TRT) can be defined, depending on some operating and climatic conditions.
The Swedish District heating system in Skogas and the Cheongju DH system in Korea were evaluated applying both the excess flow method and the target temperature method, as well as a combination of both. A number of malfunctioning sub-stations could be identified and improved in both systems.
Economic incentive Extended improvement work over longer periods means costs. However, it is also shown in this project that the work for decreasing return temperatures has a positive pay-off . For example, it is estimated for Sweden that the decrease of the return temperature by one degree corresponds to an economical value of the order of 100 million US$ . In certain cases, payback times for undertaken measures are in the order of months or a couple of years. It is obvious that the consequent improvement of operational temperature differences in district heating systems and resulting low return temperatures has an economic potential which is highly underestimated by many district heating operators.
ZW Energiteknik AB, Heimo Zinko
ZW Energiteknik AB, Sweden
Fraunhofer Center for Energy and Environment - FCEE, USA
Korea District Heating Corporation, Seoul, South Korea.a