Zero energy buildings and mismatch compensation factors

Abstract

This paper takes an overall energy system approach to analysing the mismatch problem of zero energy and zero emission buildings (ZEBs). The mismatch arises from hourly differences in energy production and consumption at the building level and results in the need for exchange of electricity via the public grid even though the building has an annual net-exchange of zero. This paper argues that, when looked upon from the viewpoint of the overall electricity supply system, a mismatch can be both negative and positive. Moreover, there are often both an element of levelling out mismatches between individual buildings and an element of economy of scale. For these three reasons mismatches should be dealt with at the aggregated level and not at the individual level of each building. Instead, this paper suggests to compensate the mismatch of a building by increasing (or decreasing) the capacity of the energy production unit. Based on historical data for the electricity supply area in western Denmark, this paper makes a first attempt to quantify mismatch compensation factors. The results indicate that such compensation factors are a little below one for buildings with photovoltaics (PV) and a little above one for buildings with wind turbines.

Introduction

The design and perspective of low-energy buildings have been analysed and described in many recent papers [1], [2], [3], [4], [5], [6] as well as concepts like zero carbon [7], [8], zero emission and zero energy buildings (ZEBs) [9], [10], [11], [12], [13], [14], [15]. A ZEB combines highly energy-efficient building designs, technical systems and equipment to minimize the heating and electricity demand with on-site renewable energy generation typically including a solar hot water production system and a rooftop PV system [16]. However, heat pumps and small micro-CHP units, preferably based on biomass fuels, have been taken into consideration as well.

A ZEB can be off-grid or on-grid. The main difference between those two approaches is that the off-grid ZEB is not connected to the utility grid, and thus it does not purchase energy from the external sources [17], [18]. In other words, the building offset all required energy by producing energy from RES. The on-grid ZEB is also an energy producing building, but with the possibility of both purchasing energy from the grid and feeding excess energy production back to the grid to return as much energy to the utility as it uses on an annual basis [16].

For the grid-connected ZEBs, the combination of a reduced demand and on-site production of heat and electricity to reach zero raises the issue of the hourly mismatch between demand and production and how to deal with such mismatch. How should one deal with the problem that a building combining conservation with e.g. PV may have a zero net energy input on an annual basis but at the same time exchanges huge amounts of electricity with the public grid? Should a mismatch be compensated for within the building itself or should the problem be solved at the aggregated level?

Means of measures at the individual building level could be either flexible demand or the use of energy storage [19], [20], [21]. This paper argues that one should not try to deal with the mismatch problem at the individual building level. Such problems are better dealt with on an aggregated level. Trying to solve it at the individual level is not economically feasible compared to the aggregated level. Moreover, one risks making things worse. Of course the measure “flexible demand” should be carried out at the building level but the aim should not be to level out the mismatch of the individual building in question. Instead, flexible demand should aim at contributing to the compensation of the aggregated mismatch of many buildings.

In order to be able to include the mismatch when defining zero energy, this paper suggests calculating a mismatch compensation factor identifying how much to increase the production unit in order to compensate for the influence of the mismatch on the electricity supply system outside the building. It should be noted that such influence of a mismatch does not necessarily have to be negative and consequently, situations in which the production unit may be decreased can also arise.

The size of the mismatch factor is a function of two elements: on the one hand the character of the mismatch of the building, i.e. if the production unit is e.g. PV or wind based, and on the other hand the system in which the mismatch should be compensated. This paper makes a first attempt to quantify such factors by making calculations for ZEB located in western Denmark (being part of the Nordic electricity market Nord Pool) and ZEB with on-site PV and wind, respectively. The calculations are based on actual measurements of the PV and wind production of many units at the aggregated level taking into account the levelling out between the many units.