In the Greater Copenhagen most of the buildings are heated by district heating. This means a great responsibility for the district heating companies to deliver a high quality of heat whereas the reliability of the heat supply is one of the more important issues.
In Copenhagen the district heating supply chain is horizontally divided into heat producers, heat transmission companies and, finally, the heat distribution companies.
This article is focusing on the heat supply reliability in the heat transmission system VEKS, supplying heat to 19 local heat distribution companies in the Western part of Copenhagen. VEKS buys heat from waste incineration plants, combined heat and power stations and a number of local heat only boiler stations, primarily used for peak and reserve load.
VEKS’ annual supply of heat is 9,000 TJ, equivalent to the heat consumption of approximately 150,000 households. The system is interconnected to the CTR system that supplies heat to twice more households. The heat supply reliability in the two systems can be considered independently.
However, the two companies are working very close together in order to ensure that the requirements to the heat supply reliability can be fulfilled in the cost effective way.
Overall policy for the heat supply reliability
Already in the planning phase, back in the eighties, the policy for the reliability of the heat supply was defined. The policy is an integrated part of the heat sales contracts with the local heat distribution companies.
The policy distinguishes between heat supplies under “normal operational conditions”, “abnormal operational conditions” and finally “force majeure” situations.
Normal conditions means that the heat supply can cover the heat demand 100 per cent on a day with an ambient temperature of minus 12oC and an average wind speed of 7 m/s. These conditions correspond to the design criteria for buildings set up in the Danish building norms.
The system is designed to cope with normal conditions, i.e. if there are bottlenecks in the transmission pipelines and branches, restricted heat exchanger capacity or bottlenecks in the local distribution systems, the heat can be delivered from alternative heat sources (peak load stations).
The break down of the largest heat production unit (Avedoere Power Plant, Unit 1 – 330 MJ/s) is considered as a “normal situation”, which means that the maximum heat demand should be satisfied.
Abnormal operational conditions means that the heat supply should cover 77 per cent of the heat demand as defined under “normal conditions”. In other words, the heat demand can be fully covered when the ambient temperature is above minus 5oC and an average wind speed less than 7 m/s. In the Copenhagen area the monthly average temperature is about zero degrees, which means that temperatures below minus 5 oC occur only a couple of times in a year.
The 77 per cent supply should be obtained in the following situations:
- Total supply break down in the power plants (failure of 495 MJ/s)
- Rupture of main pipe on one location
- Rupture of branch pipelines
- Break down of one heat exchanger
- Break down of one peak load station
- Break down of one pumping station.
In the above mentioned situations the 77 per cent supply can be re-established within a few hours. For ruptures of minor branch pipelines the criteria for re-establishment of the 77 per cent supply is less than 24 hours. The reason for this less restricted requirement is that fewer consumers are disturbed.
The force majeure situation can be claimed in case simultaneous occurrences appear.
Actions taken
Overall design
In order to go through with the policy for heat supply reliability, one of the most import measures was the overall system planning.
The VEKS transmission system is designed as one big loop with three main fingers. The main heat production units are connected to the loop. In case of ruptures of the main transmission pipelines, the heat supply to areas with no local boiler stations is ensured by having reserve load stations connected to the fingertips of all blind-ended main pipelines. The flow in the loop can be reversed.
Before VEKS’ transmission system was established, a number of local district heating systems supplied heat to the consumers from their own boiler stations. Most of these boiler stations are today an integral part of the VEKS system. Their mission is to act as peak and reserve load boilers. Until now VEKS has managed 39 of such stations, but this number will be reduced and optimized in the future thanks to the experience gained during the last twenty years. VEKS also supply heat to a number of new areas with no local boiler stations.
Most of the boiler stations for peak and reserve load can only supply heat to the local district heating systems. As mentioned above, some of these stations also can provide heat to the transmission network at strategic points. However, some of the major stations are also connected to the transmission network.
The large number of boiler stations and their distribution all over the system give high flexibility to the heat production, thereby enforcing the heat supply reliability.
In practise, the supply reliability is even higher than described above. The overall heat supply reliability policy includes neither the capacity of the waste incineration
plants nor the surplus heat capacity, which in many situations will be available from CTR.
Pumping station
There are seven major booster pumping stations in the VEKS system - none of them separately crucial for the supply reliability.
Heat exchanger stations
The heat exchanger stations are equipped with two ore more heat exchangers in cases where there is no local boiler back up in the distribution grid. The heat exchanger stations are designed to handle a breakdown of one of the exchangers and still be able to fulfil the 77 per cent heat load requirement, either by the undisturbed heat exchanger or by the local boiler station.
Boiler stations
Substitute boilers in the local boiler stations are not required, as the heat exchanger will act as reserve in case of failure of a boiler.
Other measures
The main nerve of the VEKS operations is the SCADA system. The general philosophy is to have a full redundancy of the system.
In practical terms, this means that all vital operations in the system will be taken over by other parts of the system, in case of break downs or malfunctions. Therefore the physical location of the dispatching centre has also been doubled.
Threats against the heat security of supply
We described passive measures that ensure a high reliability of the heat supply. But what are the threats facing the heat security of supply at VEKS?
In general terms these can be described as:
- Insufficient maintenance of the system, which may lead to technical break downs
- Damage of the system caused by external parties, for instance in connection with construction works close to the VEKS’ installations
- Stoppage of secondary supplies such as fuel, power, communication, water, spare parts, etc.
- Strikes and lock outs
- Forced entry and vandalism
- Fire
- Extreme weather conditions
- Hacker attack in technical IT systems
- Terror
VEKS can not prevent most of these threats, but can minimize the risks and the consequences in case something happens; VEKS’ Board of Directors has formulated a risk policy saying:
- VEKS shall be protected against risks which may threaten the companies’ business, the employees’ safety and the environment.
- In case an accident occurs, the consequences shall be minimized, as VEKS is considered to be a steady and reliable heat supplier and safe employer.
- The risk policy will be realised in a simple, rational and cost efficient manner.
The risk policy has been implemented through a set of guidelines, including the following:
- Risk assessment
- Preparation of emergency plans
- Relation to the regional emergency department
- Maintenance policy
- Safety rules for works on VEKS installations
- Education of employees
- Technical alarms, including leakage alarms along the pipelines
- Requirements for burglary and fire alarms
- Requirement for cooperation partners
- Etc.
Apart from serving as an instrument to ensure high heat supply reliability, the risk policy - including the guidelines – offers important background information to the insurance companies. A well prepared and documented system for risk handling gives basis for optimization of the premium.
VEKS’ transmission system has fulfilled the requirement for a steady and reliable heat supply, as required of a modern district
heating company. VEKS is in the age of twenty, so more and more focus will be put on the preventive maintenance, as some components in the system have reached the age where wear and tear is visible. VEKS will continue paying full attention to its customers, ensuring a high quality heat supply and high reliability.