Smart Energy Grids – Utilization of Space Heating Flexibility

Doctoral thesis, 2017

Buildings are the largest energy-using sector in the world. Since the generation of energy is highly associated with greenhouse gas emissions, contributing to climate change, there is a large focus on reducing energy use in buildings. However, reducing the end use of energy is not an end goal in itself: it is also important to take into consideration how the energy used in buildings is generated. In most district heating systems, the conditions for generating heat are constantly changing, and the marginal cost and environmental impact can vary greatly within a single day. It is therefore of interest to take a step back and study the energy use in buildings with a system boundary that includes the energy supply system, e.g., a district heating system. The objective of this work is to study how the control of space heating in buildings can be adapted to a system boundary that includes the energy supply system in order to gain a total cost improvement. Heat use flexibility is therefore important, and two types of flexibility are studied: First, the thermal inertia in buildings can be used as short-term thermal energy storage for shifting heat use from hours with high cost of heat generation and environmental impact to more favorable hours. Second, in buildings with both district heating and heat pump as heat sources, the heat load can be shifted between them to favor the heat source with the lowest cost of generation at any given time. In order to utilize these flexibilities, there needs to be some type of control system with integration towards the energy supply system. For this purpose, hourly heat prices based on the marginal cost of heat generation in a district heating system are used as a control input. The methods used include pilot tests, building modeling, energy systems modeling, and literature review. One pilot test is focused on quantifying the thermal storage capacity that can be utilized in buildings' thermal mass while still maintaining good thermal comfort. A second pilot test in 19 multi-family residential properties focused on using hourly heat prices, based on the marginal cost of heat generation in the district heating system, as input to the space heating control system in buildings. Modeling work includes creating linear and dynamic models for thermal energy storage in buildings based on measurements from the pilot tests. These models are used in energy systems modeling to quantify the benefits of using buildings as thermal energy storage in a district heating system. The benefits of heat source shifting between district heating and exhaust air heat pump are evaluated based on a study of heat load profiles in residential buildings, hourly heat prices in a district heating network, and hourly electricity prices from the spot market. The main results include linear and dynamic models for energy storage in building thermal inertia. A rough estimate is that a multi-family residential building with a structural core of concrete can be utilized as thermal energy storage with a storage capacity of 0.1 kWh/m2 heated area, while keeping the indoor temperature variation within ±0.5°C. A potential large-scale implementation can have a major impact on the heat load in district heating systems. Simulation results show that utilizing buildings accounting for 20% of the heat load can reduce daily heat load variations by 50%in a DH system. This also comes with significant cost savings. In the district heating system in Gothenburg, Sweden, the cost of heat generation associated with heating the controlled buildings can be reduced by at least 5.5–11.0% (only counting fuel cost and net bought electricity). The second pilot test, where hourly heat prices were used as a control input to the heating systems in the buildings, showed that the control method worked, and all test buildings reduced their average price per purchased unit of heat. The achieved savings in the test are hard to evaluate due to the short test length, but if it is assumed that the test period is representing average conditions, the savings are of the same magnitude as the results from theoretical studies. Heat source shifting with hourly prices can be an economically viable method of controlling the heating system in buildings with district heating and heat pump as heat sources. Whether this is viable depends on whether there is a situation where the heat source that is cheapest shifs during periods when both heat sources are available (if the same heat source has the lowest heating cost at all hours, there is no point in heat source shifting). This mainly depends on how high the electricity price is, how much it fluctuates, the coefficient of performance of the heat pump, and how all these factors correlate to the marginal cost of heat generation in a district heating system.