A fuel saver is a control function for photovoltaic-diesel hybrid systems that reduces diesel consumption by coordinating photovoltaics (PV), diesel generators and, optionally, battery storage. PV supplies part of the load, reducing the electrical power the generators need to provide. Battery storage is not essential for this function.
Fuel Saver at a Glance
- The controller adjusts usable PV output to match the load and generator operating limits.
- Minimum loading, reverse power prevention and sufficient power reserves determine the achievable PV contribution.
- A fuel saver does not automatically enable diesel generators to be switched off.
How Does a Fuel Saver Work?
The local controller monitors PV output, electrical load, and the power output and operating status of the running generators. If battery storage is integrated, it also considers its state of charge and permitted charging and discharging power. This information determines how much PV output can be used within the system’s operating limits.
As PV generation increases while demand remains constant, the required generator output decreases. If a generator would fall below its configured minimum load or reverse power into the generator becomes a risk, the controller limits PV output. Alternatively, an available battery can absorb energy if its state of charge and power limits allow it.
When PV output drops due to cloud cover or demand increases, generators or batteries must supply the difference. This requires sufficient power reserves and suitable response times. A generator that is not running cannot provide reserve power immediately: starting and, where necessary, synchronising it takes time.
Simplified example: With a load of 100 kW and PV output of 40 kW, a generator supplies the remaining 60 kW during steady-state island operation. Losses are excluded. Whether this operating point is permissible depends on factors including generator size, minimum loading and the required reserve.
Where Are Fuel Savers Used?
- Island grids and remote industrial sites use PV to reduce ongoing diesel demand and fuel delivery requirements.
- Commercial and industrial facilities with backup power can integrate PV during grid outages, provided the electrical system supports combined island operation.
- Existing PV-diesel systems can benefit from coordinated control where uncontrolled PV generation has previously caused shutdowns or unsuitable generator loading.
What Limits and Requirements Apply?
Lower generator output does not translate into a proportional reduction in fuel consumption. A running diesel engine consumes fuel even at low electrical loads. Achievable savings depend on factors including the load profile, available solar energy, generator efficiency and the number of generators running.
Minimum loading and reverse power are different limits. A generator can operate in an unfavourable partial-load range while still supplying positive power. Reverse power occurs when electrical active power flows into the generator. Permissible operating ranges and time limits depend on the manufacturer and generator model; there is no universal minimum load.
Diesel-off operation requires another grid-forming source in an island grid, typically a suitable battery inverter. This source must establish voltage and frequency. Sufficient battery reserves and coordinated transition and restart procedures are also necessary. Conventional grid-following PV inverters alone cannot provide this capability.
Implementation requires suitable measurement points, controllable PV inverters, reliable communication and defined responses to measurement or communication failures. Fuel saver control does not replace generator protection, synchronisation or the fast voltage and frequency control of grid-forming components.
How Can Fuel Saver Be Implemented with EcoPhi?
EcoPhi offers a fuel saver feature used, for example, in a PV-diesel system in Johannesburg. The installation combines a 500 kWp PV system with two diesel generators rated at 600 kVA each.
With its simple fuel saver control strategy, EcoPhi detects grid or generator operation. During generator operation, it adjusts PV output to maintain the generator’s configured minimum load. Implementation requires suitable measurements and compatible device interfaces. Monitoring alone does not reduce fuel consumption.
In EMS Compact, the fuel saver function is intended for operation without battery integration. EcoPhi also describes a battery-assisted fuel saver strategy. The available variant and support for generator starting, stopping or multiple generators depend on the product, project configuration and device interfaces.
Fuel Saver in Summary
A fuel saver increases the usable PV contribution in diesel-supported energy systems within defined operating limits. Suitable generator loading, available reserves and coordinated control are essential. Battery storage can provide additional options but is not a basic requirement.
Frequently Asked Questions About Fuel Saver
Can a Fuel Saver be Retrofitted?
In principle, yes, provided suitable measurements are available and PV output can be controlled. Generator controls, protection arrangements and communication interfaces must support the intended operating strategy.
How Can Fuel Savings be Verified?
Measured fuel consumption is compared with a suitable reference operating scenario, such as supplying the same load without PV support. To assess only the additional benefit of fuel saver control, the previous operating strategy of the PV-diesel system serves as the reference. Differences in load profile, operating duration and, where relevant, available solar energy must be considered. PV yield alone does not demonstrate how many litres of fuel have been saved.
Is a Fuel Saver the Same as Zero Feed-in?
No. Zero feed-in limits export at a defined connection point. A fuel saver additionally considers generator operating conditions and the hybrid system’s reserves.