POWER GENERATION
engine. This is a practice adopted by a number of Trusts, going it alone to clean up their existing and/or future generators.
SCR systems
Only one design of engine after-treatment is suitable for reducing NOx – an SCR system – selective catalytic reduction. SCR is a set of hardware placed in the exhaust stream comprising of several components. Working together, including the use of a chemical agent, the equipment massively reduces the NOx figure at the tailpipe. Of course this comes at a price – typically £50,000 to £70,000 per generator set. There is also an OPEX cost, as the unit will require maintenance and consumption of the chemical agent.
Whilst SCR systems are credible solutions to NOx reduction, Trusts must be cautious in specifying them. The technologies vary, the costs are high, and there is a continuing OPEX cost. Furthermore, plantrooms must be designed to ensure that access to the SCR system for routine maintenance can be achieved safely, with permanent means to access at height.
So, what’s the future? Could diesel sets see me through to my retirement in 13 years and counting? At the moment the answer appears to be ‘yes’. Much like many other critical industries, there is a comfort in having 200 hours of fossil fuel on site. Should the world be hit with some form of Mad Max crisis bringing the wheels to a halt (sound familiar?), then Trusts will be comforted by the fuel they keep in reserve.
Natural gas
It’s worth for a moment discussing natural gas. Often overlooked, natural gas generator sets produce virtually no particulate matter (PM), and some emit less NOx compared with diesel generators by using simple three-way catalyst after- treatment. This gives them the ability to meet and exceed the emissions limits set out in the MCPD Directive. They do not require gas to be stored on site, instead taking it directly from a local gas grid network. That ticks the box
environmentally, as there is no fear of a diesel spillage on site, or managing the logistics of refuelling and regular fuel cleaning. Add to the mix the introduction
Design and delivery of a mission-critical power solution to support essential repair work at University Hospital Wishaw.
of biomethane or hydrogen into the national grid gas network, and the use of a gas/hydrogen mix becomes increasingly palatable.
So, with all that taken in to account, what is not to love? Firstly, it’s important to ensure that the right type of gas engine is used in standby generators. It must be of the ‘rich burn’ variety, ensuring that the engine is capable of dealing with the rapid start and load acceptance demands of standby power. Secondly, while HTMs make reference to CHP (combined heat and power) systems powered by gas engines, they make no reference to the use of rich burn gas engines in standby power. There needs to be a step change here, with HTM guidance leading the charge towards cleaner fuels.
On-site supplies
The argument then is raised over having the necessary gas supply on site, and/or the ability to keep the generator running if the supply is turned off for emergency fire protection purposes – all absolutely relevant questions and concerns which, as an industry and society, we need to answer. However, think of the advantages if there is a gas storage system, or a CNG
back up ability, or a re-design of the gas pipework to ensure that an acceptable volume is held to keep the generator live. Between us, with the collective engineering knowledge we possess, we should be able to find a fix, and thus start to look at the ultimate advantages of having a generator in standby or mission- critical resilient mode, working from a non- diesel source.
For Trusts that are endeavouring to do the right thing, a compromised solution would be the use of specialist fuels which purport to reduce visible smoke. Such ‘drop in’ fuels are becoming widely available, and it would appear they ‘do what it says on the tin’, which is to reduce visible smoke; however, the fuel comes at a premium, and to fully benefit from its advantages, some engines may require minor adjustments to their timing.
Battery storage
Other alternatives such as battery storage have been tabled in open discussion. Battery storage certainly relieves the site of diesel generators, but introduces a new world of environmentally unfriendly batteries, many tons in weight, concentrated in a temperature-controlled
An NER (neutral earthing resistor) replacement project at the state-of-the-art, 635-bed, NHS Peterborough Hospital.
74 Health Estate Journal September 2021
Supply and installation of the standby power solution for Liverpool Heart & Chest Hospital.
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