District heating
Principle
A heat network, also known as district heating, supplies central heating on a city-wide scale.
These networks can supply heating, hot water or process energy to multi-tenant buildings (public or private properties, residential, commercial, industry). Industrial customers can receive steam, superheated water, etc.
District heating is generally operated by public authorities. Heat is transported in the form of hot water or steam via underground conduits similar to the electricity grid and gas supply networks. Customers establish a connection to the network to receive heat.
All sorts of energy resources can be used to produce the heat:
• renewable resources (biomass, geothermal energy)
• heat recovery (household waste incineration)
• conventional energy (gas, heating oil, coal, electricity).
District heating is clean, efficient and economical. Heat is produced on an industrial scale in large-capacity installations (heat production units or boilers) with optimally controlled combustion and control of atmospheric emissions. District heating plants often produce both heat and electricity, designated as combined heat and power (CHP) or cogeneration. In the CHP process heat that would otherwise be wasted is used to generate electricity. This method achieves energy savings of up to 30%.
Explanations
1- Operation
Combustion plants (boilers) produce heat for the network.
Many different energy resources can be used to produce the heat: Fossil or renewable resources
Heat can be produced by conventional boilers with burners, gas engines or gas turbines.
The heat is transported in three distinct forms:
- Hot water, limited by regulations to no more than 110°C.
- Superheated water, which is water at 180°C maintained in liquid state under high pressure (as in a pressure cooker).
- Steam: Old networks were designed with the principles and techniques of maritime heat production, using steam. Steam networks do not require fluid circulation pumps, unlike water networks.
A heat network is a closed circuit: underground conduits transport heat to customers, and cooled water returns to the plant where is it is heated again. Water circulation is ensured by pumps. The network is a large loop. Water circulates at high temperature, between 70°C and 180°C; to minimise heat losses conduits are heavily insulated with a thick layer of rock wool and enclosed in a protective sheath.
There are two types of network:
- the primary network: This is the part of the network between the heating plant and building delivery points (substations).
- the secondary network: This is the heat distribution network inside buildings, that carries heat from the delivery point into individual housing units or offices.
Heat is delivered to customers at different delivery points (substations) within the network.
The role of the substation is to:
transfer heat from the primary network (between heating plant and delivery points) to the secondary network (heat distribution network within the building),
adjust temperature and flow rate to user needs,
record the amount of heat consumed.
Heat is transferred between the primary and secondary networks either via a mixing tank, or through a heat exchanger.
Mixing tank: The same water circulates in the primary network and the customer building network, without separation.
Heat exchanger: The primary and secondary networks are second loops, and the circulating fluids are not mixed. Heat is transferred from one loop to the other via a primary heat exchanger.
A secondary exchanger between the secondary network and the water supply (third loop) heats sanitary hot water.
Heat transfer is measured and recorded by a certified heat meter that is verified annually. Meter readings are taken monthly for billing purposes, and consumption data analysed to detect any anomaly.
2- Organisation
Partners in district heating operations are:
==> the local authority, which decides to build the network The authority may operate the network directly, or delegate operation to an operator (public service concession, the most common form of operation);
==> the operator, the company that runs the heat network. In the case of a concession contract the local authority generally shares network management with the private operator;
==> subscribers, customers and users of the network.
Implementation
- Gather information from local authorities to find out whether district heating exists or is planned in your area, what entities are involved, how the project is structured in legal terms, types of energy used, and the scope of the network.
- Determine whether the establishment's heating system is compatible with district heating, and what investment might be needed.
- Evaluate potential savings in energy consumption and costs, system upkeep and maintenance. Actual benefits vary greatly from one system to another.
- Work with consultants and contractors qualified for district heating projects.
Costs
It all depends on the heating system in place and work required to make it compatible with the district heating network.
Benefits
- Energy expenditure is controlled
==> District heating offers users a stable price for heat over time, for the most part disconnected from fluctuation in oil prices.
==> The concession contractor is responsible for equipment installation, upkeep and replacement, for producing and transporting the heat.
- Safety and reliability
==> No fuels are stored within the buildings connected to the network. Substations generate no noise or odours. There is no fire risk or risk of carbon monoxide poisoning.
==> Equipment maintenance The state of network conduits is monitored at all times using advanced technology, to anticipate any incident or anomaly. Regular inspections are carried out, considerably reducing dioxide emissions, for example.
- Economical heating
==> The subscription cost is the same for all subscribers, calculated and negotiated to be as advantageous as possible: the more buildings are connected, the less expensive is the subscription.
- An environmentally friendly solution
==> In France, heating represents 2/3 of energy consumption in housing units, and 1/3 of CO2 emissions. District heating is one way to address these issues.
==> A healthier atmosphere: district heating plants are more strictly regulated than conventional boiler plants, and their emissions are monitored continuously. Consequently district heating networks emit fewer particles and greenhouse gases than individual or even building boiler plants.
==> Energy savings: for an equal amount of fuel consumed, district heating
has a higher output than a group of several individual or building boilers. District heating plants can use renewable energy from biomass (fuelwood), geothermal resources, the sun, or waste incineration.
Constraints
- High upfront costs, depending on the installation
- Technical constraints in relation to the existing heating system, to be examined