Backup diesel generator: when it is only backup and when it is already a regular emission source

Why to address a diesel generator already at design stage

A diesel generator is often labelled in a project as a backup source of electrical energy. The operator therefore frequently assumes that ordinary emission obligations do not apply. That may not be true.

From the perspective of air protection it is a reciprocating combustion engine. If its total rated thermal input reaches the decisive threshold, it may be a listed stationary source under the Air Protection Act. In that case it is necessary to address the operating permit, records of operating hours, testing regime, and possibly the method of determining emissions.

The greatest difference is between a genuine backup source and a source that is called backup but in practice runs regularly for other purposes. If a diesel generator serves peak shaving, electricity production, trading in regulation energy, or regular operation outside emergency situations and tests, the authority may require stricter assessment.

What backup source means

A backup source can practically be regarded as equipment intended for the event of outage or extraordinary need for energy supply. Typically this means operation during public grid failure, regular technical tests, service trials, or readiness verification.

It is not enough, however, to write in the project that it is a backup source. According to the current interpretation, the fact that the source serves as a backup energy source must be stated in the operating permit. If this is not in the permit, one cannot automatically assume that the benefits of the backup source regime apply to the source.

The operator should therefore check not only the equipment technical data sheet but mainly the operating permit, or the draft application. It must be clear that the source is being permitted as backup and that operating conditions are clearly set.

In practice: If you use a diesel generator only during outages and regular tests, monitor operating hours and have this clearly described in the permit. If the source also runs for economic operation or regulation services, it may no longer be ordinary backup.

The 0.3 MW threshold: when air protection starts to apply

For a diesel generator it is necessary to monitor rated thermal input in fuel, not just electrical output. Electrical output is usually stated in kVA or kW, but for classification under the Air Protection Act thermal input in fuel is decisive.

Annex No. 2 to Act No. 201/2012 Coll. classifies fuel combustion in reciprocating combustion engines under code 1.2 from total rated thermal input of 0.3 MW. For larger installations the total input of several engines is assessed if they are technically or operationally linked.

A typical problem is that the operator relies only on electrical output. For example, a diesel generator with electrical output of 250 kW may have significantly higher thermal input in fuel. Without the engine technical data sheet or data from the supplier the source cannot be correctly classified as falling under the listed source regime.

Operating hours: 300 hours and 500 hours

For backup sources two thresholds are especially important: 300 hours and 500 hours.

The threshold of 300 operating hours per year is important mainly for applying exemptions from specific emission limits for reciprocating combustion engines and for certain reliefs regarding expert assessment and dispersion study. If a backup diesel generator operated on liquid fuel does not exceed 300 hours per calendar year and has total rated thermal input up to 5 MW, the scope of supporting materials is usually simpler.

The threshold of 500 operating hours per year is important for the backup source regime under the Air Protection Act. It is assessed as a rolling average over a period of three calendar years. If a backup source exceeds this threshold, it must be expected that it may be viewed as a more regular operational source, not just backup.

Operating hours must be demonstrable. An estimate is not enough. The operator should have an operating log, electronic record from the control system, or other credible evidence.

Testing a diesel generator

Testing is normal and operationally necessary for a backup source. It must, however, be reasonably set and recorded. The authority may be interested in how often tests are carried out, how long they last, at what load, whether they take place during the day or at night, and whether they are counted in operating hours.

Operating hours usually include not only emergency runs during outage but also regular tests, service trials, and trial operation. Testing alone may account for most of the diesel generator's annual operation for some operators.

For data centres, hospitals, and waterworks facilities it is advisable to have the testing regime clearly described in operating documentation. If testing takes place every week, for example, it is necessary to calculate how many hours that represents per year and whether the source still fits within the backup regime.

When an operating permit may be needed

If a diesel generator falls under a listed stationary source according to Annex No. 2 to the Air Protection Act, it is necessary to verify the operating permit or its change. This applies especially to sources from 0.3 MW total rated thermal input.

For a new source it is addressed whether it should be part of the building project and whether a binding opinion and operating permit will be needed. For an existing source it is necessary to check whether it corresponds to actual state: engine type, input, fuel, location, exhaust, operating regime, and number of hours.

Special attention deserves the situation where there are several diesel generators in a facility. In a data centre or hospital there may be several units that individually appear as smaller sources but together form a significant combustion source. Then it is necessary to address summation of inputs and the entire operating regime.

Expert assessment and dispersion study

For listed sources expert assessment and sometimes also dispersion study are commonly addressed in permitting proceedings. For backup sources up to 5 MW on liquid or gaseous fuel that do not exceed 300 hours per year, however, the Act provides reliefs from the obligation to submit expert assessment and dispersion study. The regional authority may nevertheless request certain supporting materials with regard to local conditions.

In practice it depends mainly on exhaust location, proximity of residential development, number of generators, total input, testing regime, and whether the source can affect immission situation. One situation will apply to a single smaller generator in an industrial site and another to a set of generators in a data centre near residential buildings.

If a source is to run more often than just as backup, it is advisable to expect that the authority will want more detailed emission supporting materials. For larger sets of diesel generators a dispersion study may become a practically essential supporting material for defending the project.

Emission measurement or calculation

For genuine backup sources it is important to distinguish whether the obligation to determine pollution level applies at all and, if so, whether it is carried out by measurement or calculation.

If a backup diesel generator is operated less than 300 hours per year and no specific emission limits beyond the decree are set for it in the operating permit, the operator may have no obligation to determine pollution level either by measurement or calculation. If, however, the source is operated more than 300 hours but still meets the backup source regime up to 500 hours as a three-year rolling average, pollution level may be determined by calculation.

This is a fundamental difference compared to a regular operational source. For a source that is no longer genuinely backup, obligations may be stricter and authorised emission measurement may be required according to the permit or implementing regulations.

More on emission measurements can be found on the NATURCHEM Emission measurement page.

Exhaust, chimney, and source location

For a diesel generator the technical design of the exhaust is also important. The flue gas outlet should not discharge into a place where persons may be disturbed, flue gases may be drawn into ventilation, or surrounding windows and facades may be excessively loaded.

For new installations it is advisable to verify exhaust height, distance from ventilation air intakes, surrounding development, and possibility of safe access for potential measurement. If the measuring point is forgotten in the project, subsequent solution may be difficult.

A diesel generator is moreover often located in a container, basement, or technical space. Besides emissions, exhaust noise, air intake, cooling, fans, and transmission of vibrations into the building structure must therefore also be addressed.

Diesel generator noise

Even though the article mainly addresses emissions, noise cannot be overlooked for a diesel generator. Short tests may be very noticeable in the surroundings, especially if carried out early in the morning, in the evening, at night, or at weekends. For hospitals, office buildings, data centres, and industrial sites near residential development the regional public health authority (KHS) or building authority may require a noise study.

The operator should mainly address the exhaust silencer, intake and cooling air exhaust, container acoustics, testing operating hours, and nearest protected spaces. If regular testing regime is planned, it is advisable to set it so that it does not unnecessarily provoke complaints.

For larger sources or a set of generators we recommend assessing noise before installation. Subsequent soundproofing is expensive and sometimes technically complex.

Operating records

Records of operating hours are key for a backup diesel generator. It is from these that it is demonstrated whether the source remains in backup regime or is already approaching a regular operational source.

Records should distinguish:

• emergency operation during outage,
• regular tests,
• service trials,
• trial operation,
• operation for other reasons, for example regulation or economic.

For each start it is advisable to record date, start and end time, reason for start, load, fuel consumption, and any note on fault or test. If there are several generators, records must be kept for each source separately and it must also be possible to evaluate total simultaneous operation.

Most common mistakes from practice

The most common mistake is assessment according to electrical output instead of thermal input in fuel. Without this data the source classification cannot be reliably determined.

A second frequent mistake is the assumption that every diesel generator is automatically a backup source. If the source runs regularly for purposes other than backup and testing, its regime may change.

A third problem is missing records of operating hours. The operator then cannot demonstrate whether 300 or 500 hours were exceeded.

Absence of clear description in the operating permit is also common. If it is not stated that the source serves as a backup energy source, it may be difficult to apply the backup source regime.

Another mistake concerns the exhaust. The outlet is technically functional but discharges unsuitably towards a facade, windows, ventilation air intake, or into a space where it may disturb the surroundings.

What supporting materials to prepare

For a quick assessment the most important are the diesel generator technical data sheet, electrical output, rated thermal input in fuel, fuel type, source location, exhaust height and position, operating hours, and description of testing regime.

What you can send us for assessment

Send us the diesel generator technical data sheet, data on output and input, fuel, number of generators, location, exhaust height, operating hours, testing regime, and any existing operating permit.

We will verify whether it is a listed stationary source, whether the backup source regime can be applied, whether an operating permit, expert assessment, dispersion study, emission calculation, emission measurement, or noise study will be needed.

For unclear cases we recommend assessing the source in advance, especially if it concerns a data centre, hospital, waterworks facility, industrial site, or operation near residential development.

Brief summary

A backup diesel generator is not automatically without obligations. Decisive are its rated thermal input, number of operating hours, purpose of operation, number of generators, exhaust location, and what is stated in the operating permit.

If the source truly serves as backup and does not exceed 300 hours per year, the regime may be simpler. If it runs more, or if it is used for regular economic operation, regulation services, or long tests, it must be assessed more strictly.

The operator should have clear records of hours, described testing regime, and verified permit. These three things often decide whether a diesel generator is defensible as a backup source.

Factual basis of the article

From these supporting materials it follows that a backup diesel generator must be assessed according to actual operation. Commercial labelling "stand-by" or "emergency generator" is not enough. Decisive are the permitted regime, actual operating hours, purpose of start, and technical parameters of the source.