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Heat transfer, distribution and storage

Introduction

A functioning heating system requires district and local heating grids as well as solutions for efficiently storing heat. If that is what you are looking for, go right ahead and learn more!

Image Copyright: gettyimages.de/Ralf Müller/EyeEm

District heating tunnel

© gettyimages.de/Ralf Müller/EyeEm

In many countries, the provision of heating constitutes a large portion of overall energy consumption. In 2015, over 50 percent of global energy consumption was attributable to this area (see IEA). Some 60 percent of this share was consumed by industry (including agriculture), while buildings accounted for the remaining 40 percent. This sector, in addition to the electricity sector, is of central importance to the realisation of a sustainable energy supply. Due to its increasing integration with the electricity sector (power-to-heat), the heating infrastructure will become even more important to the energy system.

With a flexible heating system, a balance can be achieved between demand and generation, which are frequently separated by time and location. Heating grids often serve to connect large heat generators with a large number of consumers, thus guaranteeing optimal usage of the heat. Waste heat from industrial processes can also be made available in this way. In addition, corresponding grids can be used to make cooling energy available.

However, as local and district heating networks, in contrast to power grids, are only well-established in isolated regions, decentralised heat accumulators have an important role to play. These can be used to supply heat from fluctuating sources, such as solar thermal energy.

German companies have the expertise required to act an international providers of systematic heating supply solutions for the future.

To ensure an efficient heat supply, local and district heating networks as well as heat stores are needed, especially for applications and processes with high temperatures for industrial sites. This also applies to the supply of densely populated metropolitan areas.

Heating networks can help boost efficiency in the energy supply, in particular by using combined heat and power plants or by using unavoidable industrial waste heat as heat sources.

In addition, sector coupling technologies such as power-to-heat can be used to maximise the use of existing infrastructure and energy from renewable sources for an efficient and climate-friendly energy supply (see sector coupling technologies).

Heating networks

Heating networks are used to transport and supply heat, cold or steam. Depending on their reach, they are categorised either as local or district heating networks. They usually connect several heat sources with several consumers via underground pipelines or overhead lines.

Using the heat sources or heat generator, a heat transfer medium is heated to a certain temperature and transported to the consumer via the pipeline system by means of pumps. The heat transfer medium is usually water or steam.

At the consumer site, the heat transfer medium is either used directly via heaters or transferred via a heat exchanger to a second heat transfer medium, which can then be circulated and used for various heating purposes within an object like a building, for example. In both cases, the heat transfer medium cools down and is returned to the heat source in the heating network, where the cycle is repeated.

Depending on the field of application, type and number of consumers connected to the heating network and their distance from one another, the heating network must be able to meet a certain demand for heat and also guarantee a particular temperature level for heat transport.

Especially high temperatures are achieved, for example, during heat generation in fossil-fuel heat stations or cogeneration plants and are needed for process heat supply in industry. Low-temperature sources that are used within bidirectional heating and cooling networks are, for example, the waste heat from industrial processes and the heat generated from renewable energy systems such as solar collectors, biogas plants or near-surface geothermal collectors.

Heating networks can also be used to provide cooling, referred to as cooling networks. Cooling can be produced by using absorption chillers, using heat energy. In this process, the absorption chiller can either be used once heat is transported through the pipe system to the consumer site or it can be connected directly to the heat source (see waste heat utilisation).

In low-temperature bidirectional thermal networks, unlike high-temperature heat networks, less heat is lost to the environment since the temperature of the transported heat corresponds to the ambient temperature of the pipes. Bidirectional thermal networks require flow temperatures of only 40° C. In contrast, conventional heat networks in the high temperature range require flow temperatures of around 110° C in winter. In the pipe system of the bidirectional thermal networks no insulating layer is used, since the water draws on the temperature of the environment, for example in the soil, to maintain its own temperature.

Since the insulating layer is not needed, cheaper materials can be used for the pipe system. To heat the heat transfer medium to the required higher temperatures for various fields of application, bidirectional thermal systems need to use local, decentralised heat generators at the consumer site. Heat pumps, renewable energy systems such as solar collectors and waste heat from industrial processes or CHP plants are especially suitable for this purpose. The operation of bidirectional thermal systems relies on large volume flows: pipes with a larger pipe internal diameter are therefore necessary.

Thermal solar systems are suitable for supplying heat from renewable energy via local heating networks. These solar systems can be connected to a heat network in combination with a peak load boiler, biomass cogeneration plant or a heat pump as heat sources.

Well-insulated underground water pits, geothermal heat pumps or aquifer storage fields are used to store solar heat seasonally. Solar thermal systems also need large solar collector panels for heat generation: large free open spaces or extensive roof surfaces must be available on site for the installation of these panels.

Thermal energy storage systems (also referred to as heat or cold storage systems, depending on the application) are used to make the heating and cooling supply more flexible and thus allow the deferred or off-grid provision of heat and cooling. They offer great potential for a steady supply of heat or cooling, especially when renewable energy is used, since the energy supply then varies over the course of the day and year and depends on the given external conditions.

Thermal storage systems are also suitable for the use of waste heat in industry, which is flexible timewise and demand-driven. For example, the waste heat generated during industrial processes can be stored and used at a later stage for room heating and air conditioning or to supply process heat and cold. During cooling, the stored waste heat is converted into cold via absorption chillers or conventional compression refrigeration units (the waste heat utilisation).

Thermal storage can be split into three subcategories: sensible heat storage, latent heat storage and thermochemical storage. For more detailed information, please consult the brochure Energy solutions - made in Germany.

Temperature level of heat stores

Like heating networks, heat storage systems are differentiated into low-temperature and high-temperature systems. In the low-temperature range, hot water tanks are the most commonly used systems. For high-temperature storage systems, fluid storage based on liquid salt, solid storage, steam storage or latent heat storage is used.

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