Demand side response (DSR) in district heating
Written by Sonja Salo. Published earlier by Fourdeg Oy at Mon 10 Sep 2018 11:00:00 AM EEST
This article was previously published in Kuntatekniikka (12/2016)
Equipping the heating system of buildings with intelligent control devices can bring significant economic and environmental benefits. Once the buildings are connected to the local district heating network of cities or municipalities, the buildings should not be optimized individually, but the entire district heating network as a system that simultaneously optimizes production, distribution and consumption.
About half of Finnish homes are connected to district heating. Total energy consumption in 2015 was about 33 TWh, down 5 percent from the previous year, due to record warm weather.
District heating has been considered the most cost-effective way to heat buildings, especially in urban areas. Finland, like other Nordic countries, has some of the world's most comprehensive district heating networks. However, alternative forms of heating are evolving at a rapid pace. Increasing competition and changes in the direction of energy policy are putting great pressure on district heating companies to change.
District heating is currently produced directly to meet consumer demand and at fairly rigid pricing. District heating consumption varies both seasonally and daily according to outdoor temperature and consumer behavior. Flexibility in energy supply is mainly achieved through changes in production, the utilization of large water tanks and the optimization of the distribution network. Momentary peaks in demand often have to be replaced by fossil fuels because they are suited to rapid changes in heat production.
The heat gets stored in structures
Alternatively, the energy consumption of individual buildings could flex downwards during momentary consumption peaks. The purpose of district heating demand side management is to transfer the right amount of thermal power needed by buildings over time, while optimizing the total energy demand of the network. For example, thermal energy can be supplied to buildings a few hours before a peak in demand. The energy supplied is stored either in hot water tanks or in the structures of the buildings, thus reducing the energy demand of the buildings during the peak demand. This avoids the start-up of an expensive back-up power plant.
The demand side management for district heating has similarities to the one used for electricity. Internationally, Demand Side Management is also understood as total energy savings, and Demand Response in electrical systems refers to the transfer of power over time. Unlike electricity, district heating is forced to be produced locally.
District heating is also more rigid as a form of energy, and it reacts more slowly to changes and flexibility must be planned in advance. With intelligent temperature control, the end user does not notice the momentary change in heating due to the slow response.
The ultimate goal of demand side response (DSR) is to achieve savings in both energy consumption and costs without sacrificing ease of use. Therefore, domestic hot water and district heating demand side management has only limited possibilities.
Virtual thermal battery
Buildings connected to district heating can be part of a virtual, distributed heat accumulator with intelligent control. These local small thermal storages appear to the district heating company as one large, virtual thermal battery that can be charged and discharged by the operator in the same way as a large water tank heat battery.
A distributed thermal storage system works like a virtual power plant in electrical systems. In this way, a district heating company can increase its virtual battery capacity with a relatively small investment without owning any physical storage.
The distributed thermal storages can be managed remotely with devices connected to the Internet of Things. Its advantages include low investment costs, predictive maintenance and efficient use of space. A distributed thermal storage in a district-heated property is a substantial heat accumulator.
The heat accumulator charges and discharges into the structures according to the change in room air temperature, enabling a short-term heat storage of about a few hours.
When radiators increase power, the room air heats up first due to lower thermal capacity. Thereafter, thermal energy is slowly transferred into structures of the room, such as the walls, floor, and ceiling.
Correspondingly, as the power of the radiators decreases, the room structures slowly transfer thermal energy into the room air while maintaining a constant temperature.
Thermal storage capacity can be increased
The storage potential of structures depends to a large extent on the thermal capacity of the material, its mass and the thermal gradient to which the storage is applied. The buildings connected to the demand side management should be heavy structured and thus own high thermal capacity.
Thermal storage capacity can be increased by adding latent heat storage (LHS) instead of sensible heat storage (SHS), which can be implemented either by active systems or by passively integrating the phase-change materials into the structures.
Equipping a building with intelligent controls reduces a building’s energy consumption as controllers balance the interior temperature of the rooms, lower the temperature during absence, and learn the individual thermal capacity of the building. With the right control, the building optimizes and saves energy.
With demand side management, the internal temperature of a building may rise momentarily, causing heat losses to rise. The building may therefore consume more energy than it saves during the heat outage. However, this increased energy demand has been realized during a cost-effective period and the total cost of energy production may still be decreased.
Power can be cut by up to 30%
The storage potential of structures largely depends on the thermal capacity of the material, its mass and the thermal gradient to which the storage is applied. The buildings connected to demand elasticity are as heavy as possible, i.e. they have a large thermal mass.
Thermal storage capacity can be increased by choosing latent heat storage instead of sensible heat storage, which can be realized either with active systems or by passively integrating phase change material into structures.
Equipping the building with smart control devices reduces the building's energy consumption, when the controls balance the interior temperature of the rooms, lower the temperature during absence and learn the individual thermal resistance of the building. With the right control, the building saves energy as a whole.
With demand elasticity, the internal temperature of the building may rise momentarily, causing heat losses to rise as well. The building may then consume more energy when it saves heat during a power outage. However, this increased energy demand has been implemented in a cost-effective time, and the total production costs may thus decrease.
The peak power can be cut by up to 30%
Both room-specific and network-wide simulations have been made for district heating demand flexibility. The room simulation showed that the building's weakest points are in the corner rooms, which have a greater heat loss than the rest of the building. Therefore, with room-specific heating control, it is possible to utilize the storage potential of the entire building.
According to research conducted in Jyväskylä, it is possible to cut the power of the district heating system by up to 25-30%, which means that the use of a heating center that runs on fuel oil, for example, can be avoided.
The demand flexibility of district heating is part of the intelligent district heating network of the future. Based on the network simulation, a district heating company can save about 5-10% in the variable costs of district heating by utilizing a distributed thermal battery.
The share of variable costs in the total costs of district heating production varies based on, for example, the production structure, the size and age of the district heating network. In the simulation, the dependencies of the combined electricity and heat production plant and the large heat pump on the spot price of electricity were taken into account.
An energy company can find added value to the existing business by opening a conversation with the customer. This leads to one of the central questions of demand elasticity: how do we attract consumers to participate in elasticity? While the technology exists, the right economic and social incentives are still being developed.
Consumers' attitudes about endless energy supply should also be updated. The public sector could be a trendsetter and pioneer for cleaner, smarter and more efficient district heating.
Sources:
Energiateollisuus (2015) Energiavuosi
Sonja Salo (2016) Predictive Demand-side Management in District Heating and Cooling Connected Buildings,
Kärkkäinen et al. (2004) Demand side Management of the District Heating Systems
