District heating network in Helsinki Metropolitan area

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Scope

  • District Heating

District heating is efficient method to heat cities and populated areas. The base idea is to deal with heating centrally. This means customers and plants for heating are connected each other with grid. In this study it’s not purpose to examine the network precisely, only to present generally district heating and the structure of it and adjustments.

    • Need for district heating and heat output demand

Heat, which is produced by district heating, is used to heat buildings, tap water and losses in the grid. Single consumer need of heating annual demand for power depends of type of the consumer, which can classify as follows: - Maisonettes - Terraced houses - Apartment houses - Industry buildings - Business buildings - Offices, agencies - Schools. Crucial to these groups heat output demand is how and how long periods of day and week are they in use.

Need for district heating not only depend from type of the building, many other factors have effect to it. Size of the building, age, used materials and purpose of the use strongly effect consume of district heating. More significant factor is customers habits of consume heat. For example usage of warm tap water, indoor temperature and ventilation effects very strongly amount of consumed heat. In matter of single apartment district heating power output is determined by warm tap waters condenser, because the amount of heat, which is needed to warm houses indoor temperature, is small in proportion of heat, which is needed to warm tap water. Heat, which is needed to warm indoor temperature, increases in proportion of total heat output demand when apartment’s numbers increases, for example in apartment house,. Multiplies apartments heat output demand is less than demand for sum of single apartments. Difference in caused of desynchronized usage of warm tap water. [1]

    • Describing difference in demand of energy

The duration curve of heat load is used for describing difference in demand of used energy. In the duration curve of heat load is hourly heat output demand put in order by the demand. Calculations and assumptions can be made by the duration curve of heat load only in cases where demands by the hours are negligible for the solution (Energiatalouden luentomoniste 1997). These kinds of calculations are total energy and producer heat output demand assessing.

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Picture 1. An Example duration curve of heat load in Great Britain years 2005-06 [2]).

Duration curve of heat load for typical house groups will form by monitoring duration of heat load curves and searching similar curves. This way typical heat load model can be formed for each house group for certain variables. Variables can be outdoor temperature, house type and exact customer group. In picture 2 is illustrated typical heat load model for a day for a particular district heating grid part.

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Picture 2. An example from typical heat load model.

    • Plants for heating connection to the district heating network

Plants for heating connection to the district heating network can be done simplified in two ways 1) direct and 2) indirect. There are many methods to realise these, but in this case there’s no reason to get in to those any further. A short analysis from these two connection types will be enough for this study.

Direct connection means, that the same water flows in district heating network as in plants boiler. This method is mostly used in small plants. It is quite cheap to realise, but in other hand it will put some restrictions for example to fuel that is used and boilers temperatures.

Indirect connection means, that plant got different boiling water, which doesn’t flow in district heating network. These two grids are connected with condenser. There’s many kind of condensers, but the main idea is that the two water flows doesn’t physically mix together. Indirect connection is mostly used in larger steam turbine plants, but it can be used in boiler plants too.

    • District heating customers connection to the network

Customer’s connection can also do in two ways: 1) open cycle and 2) closed cycle systems. There are many implements to do these, but basic principle is needed to know for this study.

In open cycle heat transferring system part or all the water which flows in district heating network will stay in the destination. This means that needed warm tap water will be taken from district heating network and poured to the sewers after use.

In closed circle heat transferring system same water which flows in district heating network will not be consumed any kind in the destination and will return to the network wholly. This means there’s condenser between the customer and district heating grid, which will transfer needed heat to customer.

    • District heating networks heat output and adjustments

District heating heat output has to specify in the building phase. Heat output can be accurate or estimated. Mostly heat output have to specified based on estimations, because there’s no accurate information of number of incoming apartments or how big of energy needs does customers have. In generally speaking heat output is dependant of flowing fluid mass flow, specific heat capacity and cooling:

Ф= qmcp∆t (1) where, qm = mass flow cp = specific heat capacity ∆t = cooling Ф = heat output.

Needed heat output is controlled mostly by controlling outgoing waters temperature in district heating. This outgoing water temperature is often put dependant to outdoors temperature. Heating up the warm tap water and pipes heat durability defines limit-value for outgoing water temperature.

District heating grid is adjusted also for some other reason than heat output demand. Grids pressure differences and static pressure have to adjust also. These needs for adjustments and their execution methods are dependant to each others.

The main district heating system adjustment factor is consumers need for heat output demand. Heat provider have to guarantee specific pressure difference and heat output in the grid that customer can get needed heat output from the grid. Unnecessary high pressure difference or heat output in the network will cause higher heat losses and thus have to be avoided. Also pressure difference in whole network has to adjust. This way flowing waters temperature will not go under boiling temperature and vaporization will not acquire in the network. Because of this, pressure in the network has to be adjusted so it won’t go under vapour pressure.

In heat-only boiler station outgoing temperatures adjusting will realised by mixing hot water from boiler with flowing water in the network until wanted temperature is reached. Pressure differences and static pressure adjusting is mostly adjusted by pumps. These pumps are adjusted by two methods: 1) by throttles or 2) by adjusting pumps tacks.

By using throttling in the heat-only boiling stations doesn’t need regulators, because consumers will adjust the flow so they will get needed heat output by their own condensers. By throttling most of the heat will be engaged to water in district heating. Because energy for pumping is used electricity, its value is higher than heats. Adjusting pump tacks is significantly cheaper than throttling. [3]

    • Heat storing

District heating consume is not steady but there’s not only short term but also long term variation. Long term variations means variations by the months and are caused changes in outdoor temperatures. Short term varieties are hourly changes and are affected by outdoor temperature and in addition other weather conditions, like wind, clouds and rain and of course change of demand for warm tap water.

These varieties will make harder for timing of district heating production, because most of the heat produced is Finland comes from a cogeneration plant, also called heat and power plant (CHP). Heat and power cogeneration plant is adjusted to match heat demand and produced power depends from that. In other words produced power depends from plants ratio of produced heat and power.

Heat-only boiler plants heat load can be reduced by storing the heat in the district heating network or separate heat storage and reduce dependence of need for backpressure from consumption of heat. Heat storing in co generated of heat and power district heating network has many benefits: - Backpressure power productions increase when recharging heat storage. - Adjustable power production capability to power grid. - Lower district heating energy production costs by storing heat when production costs are lower and discharged when costs are higher. - Replaces energy, which is lost in backpressure plant or heat-only boiler plant planned or unplanned shutdown. - Reduces need for heat output peak evening plants. - It is more environmental energy source than separate heat-only boiler plants, because emissions to air are then focused to bigger plants.

Water filled tank is most suitable for short time heat storing, where water acts as mass which binds heat and as heat transferring fluid. Water has heat storing capability of 1,16 kWh/m3,°C. Warm water will settle top of cold water in the tank, because of density differences.

Heat storages can be connected to district heating network by two ways: 1) directly, when district heating networks water will flow trough the storage and 2) indirectly, when storage has own separate water circulation system. [4]


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  1. ↑ Huovilainen, R. T.; Koskelainen L. Kaukolämmitys. Lappeenrannan teknillinen korkeakoulu. Lappeenranta 1982. ISBN 951-763-209-6
  2. ↑ National grid. 2006. GB Seven Year Statement 2006. National Grid plc. [Available at http:// www.nationalgrid.com/uk/sys_06/print.asp?chap=all, last visited January 8th, 2009]
  3. ↑ Huovilainen, R. T.; Koskelainen L. Kaukolämmitys. Lappeenrannan teknillinen korkeakoulu. Lappeenranta 1982. ISBN 951-763-209-6
  4. ↑ Sipilä, Kari. Kaukolämmön varastointi. INSKO 12-85. Insinöörititeto Oy. ISBN 951-794-500-0
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