ELECTRICAL RESILIENCE
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
Figure 3: An excerpt from HTM 06-01, clause 3.11.
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
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 Figure 4: A standby diesel generator. 54 Health Estate Journal September 2023
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.
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