California greenhouse growers can find a rotating outage group or block printed on their utility bills, a detail many skip. The number indicates where a site stands in line if the grid runs short. Growers elsewhere also have a place in that line, even when it is not printed, because federal reliability standards require grid operators nationwide to maintain emergency load-cutting plans.
A plan for minutes of warning
Utilities keep emergency plans for cutting power when supply cannot meet demand. When those plans are used, service is shut off systematically, often with only minutes of notice. Hospitals and other critical facilities are protected where possible, but almost everyone else, including greenhouses, is exposed. Rolling outages have affected several regions in recent years during heat waves and winter storms. For a greenhouse, loss of power can stop fans, pumps, irrigation, heating and controls at the worst time.
Utilities plan for such events in detail; most greenhouses do not. Ian Knisely, an energy analyst at Energy Bridge Advisors, draws a distinction between an outage and a curtailment. In an outage, everything stops at once. In a planned curtailment, the grower decides in advance which loads to drop, how much to reduce them and which must stay on to protect the crop.
Four steps to a curtailment playbook
A curtailment plan is a written playbook for hours when the grid is stressed. Knisely outlines four steps.
- Inventory major equipment. List supplemental lighting, fans, evaporative cooling, heating, pumps, dehumidification, controls and any cold storage, along with a rough estimate of each load’s power draw.
- Sort loads by crop risk. Some can be shut off entirely for a few hours without harming the crop. Some can be reduced, such as by dimming lights, staggering fans or changing a setpoint by a few degrees. Controls, alarms and whatever keeps the most sensitive crop alive must remain on. The sorting depends on the crop and its stage and can change during the season.
- Treat forecasts as a trigger. An outage order may arrive with minutes of notice, but heat waves and cold snaps are forecast days ahead and grid operators usually issue public alerts before an emergency. In summer, growers can pre-cool in the morning, irrigate early and close shade curtains before the afternoon peak. In winter, they can heat before the cold peak, close thermal curtains early and confirm backup heat is fueled and tested.
- Assign responsibility. Decide who receives the alert, who makes the decision and who carries it out. If an environmental controller can run the sequence automatically, it should be programmed and tested. A plan that depends on one person answering a phone at 4 p.m. on a Saturday is not a plan, Knisely writes.
What the flexibility is worth
After completing those steps, a grower should know how many kilowatts can be given up, for how long and at what cost to the crop. That figure is what the grid is willing to pay for. Before ordering outages, utilities and grid operators will pay customers that agree in advance to cut load, because a megawatt that disappears on request is cheaper than one bought at emergency prices. The market is known as demand response and exists in most of the country, largely so the outage plan never has to be used.
Knisely advises growers to ask their utility where a site falls in its plan, then begin making their own. The distinction is control: preparation lets growers decide which loads stop, which are reduced and which stay on to protect the crop.