Low-temperature District Heating

There has been a considerable effort made to reduce energy losses in district heating (DH) systems. This took place in many fields of research and development, including the pipes and components delivered by companies.

The reasons for reducing primary energy consumption were compelling: the high price paid for energy in Denmark during the 70s and 80s. air pollution, diminishing world energy resources, and concerns about security of supply.

One area of energy conservation is low­ temperature DH. where temperatures throughout the system are to be lowered as much as possible at all times. The system and processes must be optimised to meet the continually changing heat demand.

Some advantages of reducing DH temperatures are:
­ Reduced heat loss
­ Improved performance of combined heat and power (CHP) plants
­ Enhanced possibilities for utilising waste heat and solar energy
­ Increased use of less­stressed, cheaper preinsulated pipes

As a rule, low­temperature DH is introduced in parallel with changes in the system, removal of bypasses, investment in better performance pipes, and renewal of unsuitable consumer installations in some parts of the system. Bypasses act as short circuits to maintain supply temperature levels constant throughout the net and in the consumer installations. Removing bypasses is of crucial importance if a low return temperature is desired, because the water passing through them is not cooled down. In Danish DH systems, many bypasses have been removed and some have been rebuilt with thermostatic controls.

It has been possible to lower the system temperature in many older Danish DH systems, partly because of considerable refurbishment of the building stock. This, along with radiator oversizing at the time of system construction, reduce the requirements for high supply temperatures.

The process of lowering temperatures will always be influenced by a prevalent conservative approach. Contracts of delivery may have to be renewed, and upcoming problems at the consumer end must be overcome through technical assistance and replacement of some parts. Attaining the optimum temperature level is a matter of weighing the day­to­day energy savings against the higher investment costs needed to lower the temperatures in the system. This demands considerable planning.

At present, the temperature levels in Danish DH systems are lower than in other countries, as a consequence of the history of the process, its technology, and cooperative efforts.

The planning of new low­temperature DH installations must comprise the entire system. Decisions concerning temperature levels must take into account plant investment, pipe investments, efficiencies obtainable, and performance. During the planning, the optimal pipe dimensions reflecting the temperature difference and eventual higher flow rate of the system must be investigated. Moreover, the DH distribution system and consumer systems must be flexible to a certain extent in order to secure the maximum benefit from future technological developments.

The Impact of Temperature Reduction

In CHP plants the ratio of power to heat output, the Cm­value, depends on the supply and return temperatures of the DH water. A reduced DH supply temperature results in a higher production of electricity and a higher overall energy efficiency. Thereby, a temperature reduction will reflect energy savings in two main areas: reduction of fuel consumption in both the electricity and heat production and reduction of heat losses in the heat distribution system.

The energy savings obtained in the heat production and distribution system depend not only on a reduction of the supply temperature but also of the return temperature. The relative importance of the two temperatures on enhancing the performance of the heat production system strongly depends on the type of heat­producing system actually used .

An analysis of the effect of a 10⁰C reduction of supply and return temperatures in the Danish DH system by 1998, beyond the already low values shown in the previous table indicates that important reductions in energy consumption are still to be expected.

Reducing Plant Fuel Consumption

In back­pressure steam CHP plants the impact of lowering the DH supply temperature is moving the process towards the generation of more power according to an increase in Cm­value. The power production can increase by 0.2­0.3% per degree of sup ply temperature reduction.

In extraction CUP plants a reduction of the DH supply temperature can increase the power production by 0.1­0.16% per degree of supply temperature reduction, depending on the existing level of supply temperature and efficiencies of turbine and power con version. Secondly, the overall efficiency will increase in terms of heat and power.

The new small CHP plants are most often gas­fired engines with Cm­values of 0.6­1.0 and overall efficiencies of 80­90%. The consumption of gas in these units will not be affected notably by reducing the supply temperature.

When a reduced supply temperature results in a higher production of electricity. it is at the same time necessary to match the heat production to the demand in the DH system, thus reflecting a higher fuel consumption and subsequently an even greater increase in power production.

An increase in power production in one plant will reflect a substitution of production in another plant connected to the power network. The overall savings therefore reflect the efficiencies in the total power production network, and depend on power production control in the network. The substitution will take place in a condensation power plant that produces only electricity with a power efficiency of around 40%. The typical efficiencies of Danish plants can be seen in table 4.

The reduction of DH return temperatures forms the basis for extracting heat from flue gas. A significant contribution to energy conservation can be drawn from the flue gas as it is cooled down and preferably also is taken through a condensation process.

Possible savings achieved by condensing water vapour in the flue gas depend on the humidity of the flue gas, and this Is a function of the fuel burned. In natural gas­fired plants condensing heat exchangers need DH return temperatures below 50⁰C in order to introduce reasonable condensation conditions. Full condensation will provide savings in fuel consumptions of up to 15%. The impact of condensing heat exchangers in plants fired by straw or woodchips will be even greater due to higher water content, however, the exchanger equipment is more difficult to construct because of problems with particles and corrosion.

A significant reduction of fuel consumption by flue gas cooling and condensation is implemented in newer production plants. For the time being, most condensing heat exchangers are incorporated in gas­fired plants. Benefits derived from using condensing exchangers is related to the technological development of corrosion­resistant ex­ changers, where possible savings that come about depend on the water vapour content of the exhaust flue gas from the system in question. Moreover, if the return temperat ure can be reduced it will greatly enhance the possibilities of using low­temperature waste heat and solar energy in DH systems.

Reducing Distribution Heat Losses

Many Danish OH systems were originally designed for supply temperatures exceeding 100⁰C. But today most of these temperatures range below 100⁰C, usually below 85⁰C. and some of them even below 70⁰C when the outdoor temperature is as low as ­12⁰C.

The total heat energy loss in Danish DH system pipes delivering more than 5 TJ per km distribution pipe length nowadays Is low. typically in the range from 12 to 20% of the distribution net input. These low loss levels have come about over many years as pipes have been replaced and net temperatures lowered. The low levels have been attained even though the replacement process is still in progress.

A small part of the total pipe heating loss is pipe hot water leakage, which can be 2% of the total heat loss from pipes, but for some systems this figure will be only 0.5%.

Decreasing the supply/return temperature levels in DH systems reduces heat losses. The temperature reduction obtainable is ultimately limited by the performance of consumer installations and is related to the performance of heat exchangers and radiators at the lower temperatures. But it is also strongly influenced by the temperature supply strategies including bypass flows.

A decrease in the heat distribution supply temperature will raise the return temperature of individual heat exchangers and double­ piped radiator systems. Nevertheless, a lowering of the supply temperature will quite often cause the return temperature to drop as well when bypass flows are present.

The performance of the consumer installations is therefore a major factor in assessing the ability of the overall system to respond with low return temperatures.

The amount of energy saved by reducing pipe heat losses depends on the amount and kind of insulation present. Based on the mean level of pipe insulation used in Denmark the savings will be approximately 0.3% of the DH net energy input per ⁰C reduction in net mean temperature.