The UK’s Department for Business, Energy & Industrial Strategy (BEIS) has developed an Electricity Supply Emergency Code (ESEC) which outlines the actions to be taken, and the timeframes, for electrical power stability recovery during an electrical power crisis. In the event of a substantial emergency, ESEC enables equal distribution of electricity supply to customers as far as reasonably practicable, while ensuring that pre-designated ‘protected sites’ maintain supplies for as long as possible.1 While the assumption is that the majority of healthcare sites would be considered ‘protected’, official recognition from their respective Distribution Network Operators (DNO) would confirm this.
Power restoration
The assumptions above are set out in more detail within the Northern Powergrid (NPG) document, Understanding emergency power cuts. Frequently Asked Questions, which states that healthcare sites classified as ‘protected sites’ are those which are ‘major hospital facilities with accident and emergency departments’.2 NPG, in its letter dated December 2022 sent to its priority customers, clarifies that no power restoration priority over other electrical power consumers is guaranteed to be provided by the service-provider, regardless of the healthcare site’s priority class. As regards power restoration to hospitals in general, the initial ‘FAQ document’ states that ‘Most hospitals have back-up generators to ensure that they can continue to operate in the event of power disruption’. A recommendation is made to review any potential risks to the site, and to have strategies in place should power cuts last longer than three hours.
Additionally, unprotected sites will be subject to the effects of a variable rota disconnection plan (VRDP), where the loss of power time window is predetermined. Power loss during rota disconnection periods equates to losing the primary electrical supply for 3-4 hours.
UK Power Networks (UKPN) has a user-friendly ‘tracker’ of annual power cut length and impact on the number of consumers, which provides insight into service reliability. The indicated average power cut length, as per the 2018/2019 period, was estimated to be 30 minutes. The level of resilience depends on the stability of the regional grid, which has to be acknowledged as an element of risk if and when innovative engineering proposals are put forward for consideration.
Resilience requirements within healthcare
Preliminary recommendations and guidance relating to electrical distribution throughout NHS sites are provided within Health Technical Memorandum (HTM) 06-01. The resilience of electrical services, alongside services redundancy, are essential to ensure continuous hospital operation during a loss of primary electrical supply (PES). HTM 06-01 provides information on decision-making as regards electrical services resilience with respect to loss of mains (LoM) creating risks to patients and business continuity. Figure 2 presents both types of risk gradings, from high to low: business continuity on the left, and patient risk on the right. The electrical resilience aspect should be considered from a final circuit resilience perspective, as well as from the whole site load perspective. When options are considered, it is paramount to abide by the requirements set out within EREC G99 and BS 7671 Chapter 82 to ensure safe operation, sustainability, and efficient use of energy.
Replacing standby diesel generators with Battery Energy Storage Systems
The next section of this article relates to the whole site (or section of the site resilience), and the proposed replacement of existing standby diesel generator(s) with Battery Energy Storage Systems (BESS). It should be noted that the level of desired resilience from a site coverage perspective, to an even greater extent than from a final circuit perspective, should be evaluated and targeted to ensure that the site can achieve the desired levels of risk reduction for a determined time period
When the primary electrical supply is lost within a hospital due to issues upstream of the electrical grid, the secondary power supply (SPS) – in conjunction with local tertiary power supplies – takes over the main and/or essential load, and ensures that there is no disruption to business continuity.
Sufficient fuel supplies on site
HTM 06-01, clause 16.12, recommends that sites equipped with standby diesel generators store sufficient fuel for 200 hours of continuous generator operation at full load. In recent healthcare development schemes this allowance has sometimes been reduced via derogation to 72 hours, on the basis that the site has an agreed contract in place with a fuel service provider that affords it priority for fuel delivery. Such departures from set requirements should be managed accordingly, as articulated within HTM 06- 01, clause 3.26
A typical BESS system installation is designed to provide site resilience for a few hours, and – should the back-up time be extended – the associated installation costs and physical space required for the system increase significantly. Regardless of the selected system, and from the distribution system design perspective, HTM 06-01, clause 4.2, states: ‘The design process should verify that single points of failure leading to loss of electricity supply are minimised by providing the appropriate level of resilience at the point of use’.
‘Pros and cons’ of standby generators and BESS systems
While the BESS and the back-up generator are ostensibly both suitable for the same function – as articulated within the definition of a Secondary Power Supply in HTM 06-01 – each has its differences, both from a short, and a longer-term operation, perspective. (see Figure 3).
Both systems have comparable requirements in terms of placement and installation. A BESS requires space for all its batteries or its containerised assembly to be located with sufficient access available around the unit. Considering the 200-hour continuous operation requirement for a site utilising 2 MW of power, it would result in a significant space on site for battery replacement. Standby generators ideally need to be located adjacent to the associated main low voltage switchgear, or – in the case of a high voltage generator – within a reasonable distance of the main HV switchgear. Fuel storage is another aspect of diesel generators – since depending on the generator rating and type, a 200-hour fuel storage tank can equate to 80 tons of fuel storage at a single location, or even more
Adequate ventilation
For a BESS, adequate and effective ventilation of the room where the batteries are to be stored is essential. Depending on the battery type, certain compounds (particularly hydrogen) leak, and increase the risk of explosion. Specialist advice is required to ensure that sufficient ventilation is provided within the room. Additionally, rooms housing a BESS should be kept at a certain temperature so as not shorten battery life. Lithium-ion batteries (LIB) for optimal performance in a BESS should be maintained at +20°C – +25°C,3 although the true operational range of LIB is -20°C – +60°C.4 Other sources suggest that the ideal LIB operational temperature range is +15°C – +35°C.5 This temperature maintenance will add additional operational carbon emissions on top of the BESS Life Cycle Assessment (LCA)-estimated figure. Standby generators have frost protection heaters and engine oil immersion heaters to ensure smooth generator operation, which also is considered an addition on top of the LCA estimate.
From an interrupted service restoration perspective, a BESS is superior when compared with a standby generator. There is no break in power delivery when LoM occurs, thanks to the high switchover capability of the UPS-like configuration. Conversely, standby generators need to get up to speed to be able to accept load. HTM 06-01 considers power loss of up to 15 seconds as an acceptable interruption to clinical risk grade A and B areas, with certain circuits connected to the local UPS system. A BESS permits a noticeable smooth transition from LoM to the generator if the two systems are arranged to work effectively in tandem
Long economic lifespan
The biggest advantage of standby generators is their economic lifespan. Standby diesel generators have an economic life of up to 25 years, during which the generator remains predominantly inactive, and only accumulates maintenance hours.6 When the generator is required to operate during a LoM emergency, assuming there is a constant fuel supply and the engine has been properly maintained, it should run for prolonged periods without any issues. HTM 06-01, clause 7.22, suggests that ‘Generator reliability may be of the order of 99.95% (or 0.05% unreliable), which equates to 4.5 hours of unavailability per year’.
Standby generators can be used for short-term operational reserve (STOR) and other Distribution Network Operators’ demand side management schemes – via which a revenue stream can be generated to the owner. Due to the carbon footprint of standard diesel fuel, generator operation to accommodate this is not a sensible solution from an environmental perspective.
Better suited to revenue generation
A BESS is better suited to revenue generation due to greater system flexibility. As previously mentioned, such systems are capable of almost instantaneous connection to the grid, which is beneficial for Dynamic Fast Frequency Response (DFFR) schemes. In addition, the BESS can be charged overnight when the grid has its lowest carbon footprint per kWh, and discharged during the day to provide demand peak shaving for the site in question. A downside of a BESS, however, is the sensitive electronics equipment incorporated within it, which regulates the system’s operation. The effects of dust accumulation over time, and incorrect battery temperature management, can render the entire system defunct.
Diesel generator conversion to biofuel operation brings additional maintenance requirements from a fuel perspective. Generator filters can get clogged up, and the injectors are at risk of damage due to ‘diesel bug’, where microorganisms present within the fuel start biomass-film formation.
To ensure 200-hour resilience provision would require a significant size of BESS to replicate what a single diesel generator can provide. In addition, if the BESS runs out of power, there is no way to charge it should the grid be ‘down’; hence why the BESS has to be sized in respect to the site resilience strategy. However, the system can be charged during the day utilising local solar PV or wind power-generated energy, resulting in reduced operational carbon footprint
Carbon footprint comparison
Let us now consider the estimated carbon footprint associated with the two different systems based on the assumed economic lifecycle and the most likely operational regime. The tables in Figures 5 and 6 set out the assumed parameters for each system. While the data for two systems highlighted in Figures 5 and 6 have been based on the same 800 kW rated capacity, the duration for which the BESS is required to provide electrical back-up resilience is set to two hours. This running duration is considered typical for a BESS installation, in contrast to the run time for a diesel generator, due to the differences between the two systems, as previously stated. The carbon emissions associated with the temperature maintenance for the two systems have been disregarded as part of this calculation.
A further calculation has been made based on the conversion of a standby diesel generator vs a biodiesel generator, in order to compare the carbon footprint of each. For this calculation, the assumption has been made that the generator consumes the same amount of B100 biodiesel as standard B0 diesel fuel. It should be noted that biodiesel has 7% less energy per volume in comparison to B0. The latter is taken into consideration within the calculations and figures I will now present.
For the carbon footprint calculation, it is assumed that the diesel generator will operate for 200 hours (no. 1 in Figure 5) every two years, in addition to its standard maintenance cycle (no. 4 in Figure 5). The indicated 200 hours of operation are also split into 100% of its rated capacity (daytime no. 2 in Figure 5), and 50% of its rated capacity (night-time no. 3 in Figure 5), for more detailed representation. The comparison does not take into consideration what the longest and shortest intervals of continuous generator operation are, which would help in making a more precise comparison of the two systems.
The following four sources of energy have been identified for the BESS unit (in Figure 7):
Charging from a wind-generated power source.
Charging from a solar-generated power source.
Charging from the grid during the best recorded month.
Charging from the grid when its carbon footprint is average.
The embodied carbon for each system has been estimated as follows:
Generator – 12,000 kg CO2 eq (based on 9,000 kg CO2 eq for a 500 kW generator)
BESS – 88,000 kg CO2 eq (based on 55 kg CO2/kWh for a Lithium-CobaltAluminium battery).
Carbon footprint lifecycle assessment of BESS
The carbon footprint lifecycle assessment for the battery energy storage system has been estimated as follows:
a) Estimation of Total BESS energy over a lifetime (TEOL) = ((BESS operational capacity) x (initial energy) x (no. of cycles)) / (efficiency rating).
b) Estimation of operational carbon footprint per lifetime (OCFPL) = (TEOL) x (source of energy carbon footprint).
c) Estimation of total carbon footprint per lifetime of use = (OCFPL) + (embodied carbon of selected BESS).
As is shown via the calculated outputs, the source of charging for the BESS is very important. The total operational carbon figure when a BESS is charged from the electrical grid utilising average carbon values per kWh equates to 1,930.7 tCO2 eq, which is significantly higher when compared with the value achieved when the BESS is charged from the wind-generated source (64 tCO2 eq). Total LCA values are presented within the ‘Total Carbon footprint per lifetime of use’ table in Figure 8.
Carbon footprint lifecycle assessment of a standby generator
The generator carbon footprint calculation for the embodied carbon and operational carbon use has been estimated based on the following two scenarios:
1 With the 200-hour generator operation calculated as a 100% rated capacity, plus the carbon footprint of the ongoing maintenance.
2 With the 200-hour generator operation split into 50% of time operating at 100% rated capacity, and 50% at 50% rated capacity, plus the carbon footprint of the ongoing maintenance.
Biodiesel generator carbon footprint (estimated by converting the quantity of traditional diesel fuel used)
Based on the defined operational parameters and the two indicated scenarios, the LCA of the analysed generator set while running on diesel was 1,321.44 tCO2 eq and 1,057.44 tCO2 eq. Carbon LCA figures were significantly lower when the generator was running on biodiesel, resulting in 386.5 tCO2 eq and 311 tCO2 eq respectively.
Conclusions
There is no guaranteed level of assurance that the Distribution Network Operator can provide during a power cut. HTM 06-01 sets out information on final decision-making regarding electrical services resilience with respect to patients and business continuity. With their instantaneous response to power loss, and ability to charge when the grid carbon footprint is lowest, Battery Energy Storage Systems certainly have a place in healthcare as another layer of ‘defence and stability’, and while carbon-emitting diesel generators are considered ‘old technology’, with their ability to operate for as long as fuel is present, they still provide further resilience and a last line of defence against loss of power when other systems fail to enable healthcare sites to continue operating.
The two systems have similar levels of carbon emissions throughout their lifetime. The carbon emissions are subject to the recharging source of electricity for BESS, and the type of fuel and frequency of use for the generator.
The importance of a fully functioning infrastructure is sometimes taken for granted. The loss of it would be devastating to the modern world. In addition to healthcare, service disruption to infrastructure, including water and drainage, due to power cuts would cause severe disruption nationwide. A relevant resilience strategy must be established to provide power for extended periods of time. This underlines the significance of the systems operating in the background while providing reassurance of service continuation
While both standby diesel generators and BESS systems are considered acceptable to ensure electrical resilience on hospital and other healthcare sites, however, the associated risks should be fully assessed. If considering swapping one for the other, a risk assessment needs to be undertaken, with an agreement and final sign-off by the Electrical Safety Group
Tomas Jucas
Tomas Jucas is a principal electrical engineer within the National Healthcare Team at Mott MacDonald. He has nine years’ building services experience, the majority having been spent honing his skills, and increasing his expertise, within the healthcare sector. He graduated with a Bachelor’s Honours degree in Electrical Engineering in 2008, and was subsequently awarded a Master’s degree in Building Services Engineering with distinction in 2021. He said: “I have developed expertise within generic electrical services in buildings, while working for some great companies, where the knowledge passed down to me has contributed to me becoming a competent engineer, and achieving Chartered Engineer and Fellow of IHEEM status in 2022. I find medium and low voltage electrical distribution infrastructure among the most intriguing aspects of engineering – a factor which has further motivated me to progress toward achieving Authorising Engineer status in association with IHEEM.”
References
1 Department for Business, Energy and Industrial Strategy. Electricity Supply Emergency Code. Published 1 January 2015; updated 6 November 2019.
2 Northern Powergrid. Understanding emergency power cuts. Frequently Asked Questions. 2023. https:// www.northernpowergrid.com/ emergencypowercuts
3 Boyer C. The importance of thermal management of stationary lithium-ion energy storage enclosures. 24 April 2019. Solar Power World Online. https://tinyurl. com/4dcc7z4v
4 Shuai M, Jiang M, Tao P, Song C, Wu J, Deng T et al. Progress in Natural Science: Materials International 2018; 28(6): 653-666. Available from: https://www. sciencedirect.com/science/article/pii/ S1002007118307536
5 Pesaran A, Santhanagopalan S, Kim GH. Addressing the impact of temperature extremes on large format li-ion batteries for vehicle applications (presentation). National Renewable Energy Lab. (NREL), Golden, CO (United States), 2013. https:// www.nrel.gov/docs/fy13osti/58145.pdf
6 Guide M. Maintenance engineering and management. CIBSE, November 2014. https://tinyurl.com/26226fdb