Determinants of energy flexibility in residential hot water systems

Abstract

With increasing proliferation levels of variable energy sources, flexible energy loads will become increasingly important to help stabilize the energy grid. Increasing electrification of heating systems means that the thermal inertia of buildings and hot water vessels can provide a ubiquitous, low cost alternative to electrical storage for providing this energy flexibility. However, it is unclear how this flexibility fluctuates with various factors such as occupant behaviour and ambient conditions in the real world. This paper quantifies the effect of different influencing factors on the energy flexibility of residential hot water systems using data from a large scale real world pilot. All the houses considered in this analysis feature identical air source heat pump hot water systems, along with 200 litre storage vessels. It is shown that ambient conditions, control algorithm and occupant behaviour, all influence the available energy flexibility of the hot water system, albeit in different ways. Available capacity for energy flexibility and the corresponding recovery periods can differ by as much as two to four times for identical storage, meaning that these differences should be taken into account during operational planning with flexible loads. Additionally, it is shown that the implemented control strategy provides a meaningful avenue to alter the available energy flexibility. The paper also highlights key differences in the way these factors influence the overall energy demand when compared to available flexibility.

Energy Use in Residential Buildings: Characterisation for Identifying Flexible Loads by Means of a Questionnaire Survey

“…A great potential for flexibility can be found in buildings equipped with electric heating systems; the impact on overall consumption depends on the climatic zone (i.e., outdoor temperature and relative humidity), on the buildings' characteristics and on the occupancy rate [15]. Likewise, a great potential for flexibility can be noted in buildings equipped with domestic hot water preparation systems; in that case the impact on overall consumption essentially depends on the intended use and the number of occupants [16]. In addition to those consumptions, there are further deferrable loads owing to the use of washing machines, dishwashers or dryers, whose incidence is correlated to the users' number [17].…”

Energy Retrofitting Effects on the Energy Flexibility of Dwellings

“…Anyway, the location of the storage system inside the dwellings remains to be explored. As a matter of fact, the DHW storage system is generally small [23] and easy to install inside the dwelling. Nevertheless, the storage system required for space heating is much larger, depending on the climate zone, the characteristics of the house, and the behavior of the occupants The results of the simulations carried out confirm the usefulness of the heat pumps to increase the flexibility of the loads [62,63], as a basic element of a system that must necessarily include storage systems [64][65][66].…”

“…Anyway, the location of the storage system inside the dwellings remains to be explored. As a matter of fact, the DHW storage system is generally small [23] and easy to install inside the dwelling. Nevertheless, the storage system required for space heating is much larger, depending on the climate zone, the characteristics of the house, and the behavior of the occupants [67] together with needed preservation of architectural appearance when the building is considered historic or even listed [68].…”

“…The share of electric-driven heating and cooling on the total electricity and whole energy demand is, therefore, linked to the climatic conditions of building location, its characteristics, and the occupants' behavior [22]. As a matter of fact, the intended use of the building is where electric DHW production is installed and number of occupants are the main parameters affecting the final load value [23]. Moreover, those parameters affect the shiftable loads as well and can be generalized to all the electric appliances and devices used in dwellings [24].…”

Interactive Buildings: A Review

“…Heat pump DHW systems use electricity to provide thermal energy to water. Hot water storage tanks enable DHW systems to provide flexibility to the grid [68]. Similar to HVAC loads, the DHW heater's operation is affected by the outdoor temperature, season and time of day.…”