Page 40 - Energize May 2021

TECHNICAL

Grid interactive, energy-efficient

buildings: the next step from “net-zero”

by Mike Rycroft, Now Media

The “net-zero energy building” has become a common way of describing the combination of energy
efficient building systems and renewable energy systems to reach a balanced energy budget over a yearly
cycle. Even though the wording “net-zero energy building” focuses on the annual energy balance, large
differences may occur between the energy balance at different times and seasons. n

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nergy efficiency equipment and building management systems Researchers have noted that while the net-zero concept is a
have helped reduce energy consumption and own generation convenient tool, it is insufficient to describe the energy performance
Ein the form of solar PV and solar water heating, and other of a building and its potential role as an active element in the energy
sources have reduced the amount of energy that a building draws network. If the building-grid interaction at smaller timescales is not
from the grid. considered, net-zero buildings could have a detrimental impact on
A net-zero energy building (NZB) is usually described as a the performance of the grid at high penetration levels.
grid-connected building which generates as much energy as it uses It has been suggested that the performance of net-zero
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over the period of a year. While an annual energy balance is useful, buildings will in future be measured by two related dynamic
a complete description of a NZB requires examining the system at metrics: the interplay between on-site generation and the building’s
smaller time scales. demand called “Load Matching”, and the resulting bidirectional
The fact that the NZB relies on the grid as an energy balance flow of energy with the grid, commonly called “Grid Interaction”.
can cause large variations in the amount of energy flowing in and The next step forward from the NZB is the “grid-interactive efficient
out of the grid. The net-zero concept does not deal with the issue building” (GEB).
of how energy flows between the building and the grid is controlled,
and a passive NZB might be at odds with the grid if too much own- Grid interactive efficient buildings
generation were to take place at a random time. Equally, the grid A GEB is an energy-efficient building which uses smart technologies
may not be able to accept the energy generated, or it may be unable and on-site DERs to provide demand flexibility while optimising
to supply the required additional energy on demand. energy cost, grid services, and occupant needs and preferences, in
To describe the building’s energy profile completely, an a continuous and integrated way. The GEB interacts with the grid
assessment should address both the real-time relationship between using grid information on tariffs, demand levels, frequency support
power generation and building loads and the resulting interaction requirements and other grid-related information. GEBs are buildings
with the power grid. 3 which are efficient, connected, smart, and flexible.
In operation, net-zero buildings effectively use the electrical grid
for energy storage, and this can cause concern for the grid operator. GEBs have the following key characteristics:
In an idealised net-zero model, an infinite grid has infinite capacity • Efficiency: Efficiency reduces both energy and demand level and
and is run by an organisation willing to accept excess self-generated minimises the use of resources.
energy at any time and at a favourable rate, or in equal exchange • Connectedness: The building has a bidirectional connection with
for electricity whenever the building needs it within the next twelve the grid, flexible technologies and building management systems.
months (known as net-metering). • Smartness: Analytics are supported by sensors and controls which
The concern comes from the fact that the real grid is far from optimise efficiency and flexibility as well as occupant preferences.
ideal and cannot absorb or supply electricity on demand and/or at • Flexibility: Flexible loads and distributed generation and/or storage
favourable rates. can be used to reduce, shift, or modulate energy usage.
When net-zero is measured over a 12-month period, it does not
give any indication of the short-term performance of the building. A
building may be net-zero on energy over a year, by using grid energy
in winter when demand is high and producing excess in summer
when demand is low. Although this yields a net-zero sum for the
building, the impact on the grid is far from desirable.
From the point of view of the grid, if a net-zero building draws
power during peak times, there will be little difference between a net-
zero building and a conventional one. Figure 1: Key characteristics of GEBs 4

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