Page 42 - Energize May 2021

TECHNICAL

Assuming an average EV has a battery capacity of 50 kWh,
20 vehicles would provide 1 MWh of storage, and 50 vehicles
2,5 MWh, so the potential for load shifting using EV batteries is
large. The only problem is that EVs are only there during working
hours and cannot be used outside of this period, although it
may be possible to arrange with the driver that the EV always
arrives with close to a full charge. 5
One example is the Durban conference centre. An energy
modelling exercise of this site proposes using energy from
delegates’ EVs during the morning peaks and recharges the
vehicles in the afternoon. 5

Green load scheduling 6
Green load scheduling is a form of short interval load shifting. In
a commercial building, heating, ventilating, air conditioning and
Figure 4: Building demand flexibility modes 2 refrigeration (HVAC&R) system, and chiller systems, controlled
by setpoints and operating on “on/off” modes could activate
concurrently, resulting in power demand spikes several times in
A form of modulation known as cycling or scheduling can be a day.
applied to the HVAC system to stagger the use of chillers to While several different approaches to load balancing exist,
ensure that they are not all running at the same time. Cycling controlling power consumption by load shifting and load
does not seriously affect occupant comfort. When more than shedding, they operate on coarse-grained time scales and do
one chiller is used, operation can be staggered to reduce the not help in de-correlating energy demand of individual items.
maximum demand. For instance, in an installation using four Load scheduling is an approach for fine-grained scheduling
chillers, the chillers are cycled at short intervals so that there are of control systems within an aggregate peak power envelope,
always three chillers running. HVAC equipment such as chillers while ensuring the climate conditions required of each system is
can also be switched off for short periods without affecting maintained within desired limits. Scheduling can be a complex
occupant comfort. process, but when applied to a combined system of chillers,
thermal and electrical storage, could ensure a reduction in
Load shifting and energy storage peak demand by preventing concurrent operation of devices.
Load shifting is not easy to accomplish in a commercial building Flexible scheduling would ensure that the schedule changes to
as most of the activity is responsive to demand or schedules and accommodate changes in weather and season.
cannot be moved in time. IT activity for instance cannot be shifted,
other than bulk back-up or printing operations. Load shifting could Modulation or frequency response
be accomplished using energy storage. Modulation is the short-term change in demand in response to
grid signals, to provide grid support services. It involves reducing
Thermal storage systems load for short periods on plant which can tolerate short power
Thermal storage used with HVAC is quite common. Thermal breaks, such as chillers, without affecting the comfort or
storage systems store either cold, in the form of ice used for convenience of building occupants. Such services would be paid for
cooling, or heat in the form of hot water used for space heating. by the grid operator. n
Energy is stored when demand is low and consumed when
demand rises. In a commercial building, this would usually be References
outside of business hours, but with the introduction of DER 1. M Barker: “The Importance of Grid-interactive Buildings”, www.
systems, surplus energy may be used for thermal storage instead automatedbuildings.com
of exporting it to the grid. 2. J Salom: “Understanding net-zero energy buildings: Evaluation
of load matching and grid interaction indicators”, 12th
Battery electricity storage systems (BESS) Conference of International Building Performance Simulation
Although many industrial complexes are installing bulk electricity Association, Sydney, 14-16 November 2011.
storage units, the practice is not extended to commercial 3. A Roth: “Grid-interactive Efficient Buildings Technical Report
buildings. Batteries can be charged from either solar PV Series Whole-Building Controls, Sensors, Modeling, and
systems or the grid and could be used to reduce the grid load Analytics.”, www.energy.gov
during peak periods. 4. RMI: “Grid interactive energy efficient buildings”, Rocky
Mountain Institute, 2021.
Interaction between electric vehicles (EVs) and the building 5. M Barker: “Modelling a microgrid in Durban”, HVACR Online,
EVs provide a new factor in the flexible energy sector. Many February 2019.
workers still rely on private transport and the average building 6. M Behl: “Green Scheduling for Energy-Efficient Operation
provides parking for several vehicles. In an all-EV situation, of Multiple Chiller Plants”, 33rd IEEE Real-Time Systems
there could be interaction between the building’s electrical Symposium (RTSS), 2012.
system and the EVs’ batteries to supplement the energy
storage capacity. Send your comments to rogerl@nowmedia.co.za

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