Power System Integration of Flexible Demand in the Low Voltage Network

Abstract

Flexible demand for electric power is expected to play an increasing role in the future power system where an increased share of the power generation will come from renewable energy sources. In Denmark,households accounts for approximately one third of the total electricity consumption; thus, it is natural to consider electricity consumption from households as an integrated part of a potential flexible demand side in the future power system. However, as an individual unit a single household is not able to generate a large impact in the power system. Therefore, a mechanism is needed which can coordinate the response from numerous of small entities: in other words, we need a tool for aggregation of a large number of small units and coordinate the electricity consumption among them. This thesis investigates the possibilities of utilising flexible demand from small household entities connected in the low voltage grid. A special emphasis is placed on control of electric space heating as a flexible resource due to the fact that power consumption for heating and cooling applications constitute a significant share of the total electricity consumption in Denmark. In a Nordic perspective the potential is even greater as almost 50% of the electricity consumption in households is utilised for space heating and cooling and for heating of domestic hot water. Consequently, there exists a significant potential in the integration of power consumption for heating purposes in households for demand response in the Nordic power system.This thesis describes how power consumption in an intelligent building can be controlled and coordinated with other entities by aggregating the unit into a so-called virtual power plant. This approach is demonstrated by tele control of the electric space heating in the intelligent building by use of the infrastructure of a commercially running virtual power plant, which has been developed by the Danish energy company DONG Energy. The virtual power plant continuously optimises the power consumption of a portfolio of flexible distributed energy resources with respect to the cost of electricity on the Nordic energy exchange, Nord Pool, or alternatively the flexibility can be sold as an ancillary service to the transmission system operator. In this way, the electricity consumption in households are indirectly integrated into the power market, through the virtual power plant, and a coupling between consumption and the cost of electricity is made.Field experiments with control of space heating show that electricity consumption for heating can be partially lowered or entirely postponed during peak load hours and thus contribute to a reduced need for transmission capacity in the distribution grid. Moreover, simulations show that at least 20% of the daily electricity consumption for heating in an aggregated unit of households can be shifted from midday to midnight, where there is typically a surplus of renewable generation.Finally, it is concluded that aggregated electricity consumption for space heating can provide fast, reliable and accurate demand response,and thus provide a high flexibility in the power system. Consequently,electric space heating offers a good technical candidate for demand response in the future power system, especially when taking into account the expected increased utilisation of electricity for space heating towards year 2035, where all forms of centralised heating based on fossil fuels in the Danish energy system are to be phased out and replaced by renewable alternatives.