EMS, BESS and markets
A battery that decides on its own
A storage system is only worth what it decides. This page says what the control system arbitrages, what pays for that arbitrage, and what conditions every euro. Without quoting a single one.
The vocabulary, once and for all
BESS, EMS: the cabinet and its brain
The BESS,Battery Energy Storage System, est le système complet : les cellules, la conversion, le refroidissement, la sécurité. C’est ce que Stellabox livre, sous le nom de Mercury. L’EMS, Energy Management System, is the control system that decides when to charge and when to discharge. On the Mercury units it sits in the same cabinet: there is no extra enclosure to add, no integrator to bring in to connect the two.
It is this control system that shaves, arbitrages and switches over, and it is what makes the site dispatchable by an aggregator. A battery without an EMS is just a reserve: it is the decision that creates the value.
The Mercury 418 carries predictive management compatible with aggregation. CESC manufacturer data.
What the control system does, second by second
The decision loop
Five steps running continuously. A setpoint is not fixed in the morning for the whole day: it is recalculated as consumption, generation and prices become clearer.
- 01
Measure
The power drawn by the site, the generation, the state of charge of the cabinets.
- 02
Forecast
Consumption in the hours ahead, expected generation, announced prices.
- 03
Decide
Charge, hold, discharge. The setpoint is recalculated continuously, not once a day.
- 04
Act
The conversion applies the setpoint, within the cabinet's power and temperature limits.
- 05
Check
The gap between setpoint and reality is measured, and corrects the next decision.
Trading, without the folklore
What pays for a battery
One cabinet can serve several uses, which stack up. Sites in this sector quote percentage gains and payback periods here. We do not have those figures for the Mercury units, and an industrial buyer carries them into their calculation: so we will not write them.
What we can say, and what is worth more than an unverifiable figure: what each layer depends on, and who holds the answer. The base of the stack reads off your bill. The top requires a country, an aggregator and a qualification.
Sell flexibility
Depending on country and aggregatorThe site commits to raising or lowering its power on request, and that commitment is paid for.
Arbitrage on the market
Depending on country and aggregatorThe control system buys energy when it is cheap and releases it when it is expensive, on wholesale prices.
Shift within the day
Depending on your contractCharge during off-peak tariff hours, discharge during peak hours.
Consume your own generation
Readable on your billThe midday solar surplus is stored, then released when the site consumes.
Avoid the peak
Readable on your billThe battery covers the power overrun instead of the grid. The meter sees only smoothed consumption.
These layers do not add up freely. The same hour cannot be sold twice, and a flexibility commitment ties up a share of the capacity, which is then available for nothing else. That is the arbitrage the control system makes, and that is what is priced in the quote, on your load curve and your contract.
Country by country
The families of mechanisms
The names change from one country to another, the families do not. Here is what to look for in your contract and with your grid operator.
| Famille | What it is | What it requires |
|---|---|---|
| Capacity pricing | The bill carries a share that depends on the power drawn, not only on the energy consumed. | Nothing beyond the current contract. It is the simplest mechanism to verify. |
| Hourly and seasonal prices | The price of energy changes by hour, by day or by season. | A contract that exposes those differences, and a load curve that lets you play them. |
| Wholesale market | Energy is bought and sold day-ahead, then within the day. | A supplier who passes those prices through, or an aggregator who carries the positions. |
| Frequency reserves | The grid operator pays for capacity held available to stabilise the frequency. | A technical qualification, a controlled response time, and capacity tied up. |
| Demand response and local flexibility | The site commits to reducing its consumption on signal, over announced periods. | An aggregator, a commitment contract, and metering compliant with the country's rules. |
| Capacity mechanism | Capacity available at times of system stress is paid for. | A certification, and rules that vary widely from one country to another. |
- Capacity pricing
- The bill carries a share that depends on the power drawn, not only on the energy consumed.
- Nothing beyond the current contract. It is the simplest mechanism to verify.
- Hourly and seasonal prices
- The price of energy changes by hour, by day or by season.
- A contract that exposes those differences, and a load curve that lets you play them.
- Wholesale market
- Energy is bought and sold day-ahead, then within the day.
- A supplier who passes those prices through, or an aggregator who carries the positions.
- Frequency reserves
- The grid operator pays for capacity held available to stabilise the frequency.
- A technical qualification, a controlled response time, and capacity tied up.
- Demand response and local flexibility
- The site commits to reducing its consumption on signal, over announced periods.
- An aggregator, a commitment contract, and metering compliant with the country's rules.
- Capacity mechanism
- Capacity available at times of system stress is paid for.
- A certification, and rules that vary widely from one country to another.
Families of mechanisms observed on European markets. Whether they are open to on-site storage, their rules and their thresholds depend on the country, sometimes on the region. Stellabox supplies the equipment and its control system: access to a mechanism is verified with your supplier, your aggregator and your grid operator.
6
Families of mechanisms that pay for on-site storage.
Their names change from one country to another, the families do not. None is open everywhere: access is verified with your supplier and your grid operator.
Observed on European markets
The meter sees only a figure. What drives it up is decided here.
Before signing anything
The six pieces to gather
01
Your load curve
Power readings at ten-minute or quarter-hour intervals, over a full year. Without it, no sizing is serious.
02
Your supply contract
The exact structure of the price: capacity share, energy share, time bands, overrun penalties.
03
The country's rules
The mechanisms open to on-site storage, and the conditions of access. They differ from one country to another, sometimes from one region to another.
04
Your grid connection
The connection capacity available, and what the grid operator requires in order to add storage to it.
05
Your aggregator, if you have one
The mechanisms it actually operates in your country, and the share of capacity it ties up.
06
Your horizon
How long the site will remain in operation, and the changes in consumption already decided.
Send your load curve and the structure of your contract: the sizing and the layers actually available to you are priced on that basis.