Microgrids for data centres: could nuclear power be an option?

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    Rolls-Royce has suggested that onsite nuclear power generation could be a viable option in the future. Ian Bitterlin discusses its feasibility for data centres

    Microgrids provide a viable power option in remote regions that cannot access primary grid systems, as well as providing an opportunity for technology users to maximise their energy plan by using onsite generation to meet a proportion of their energy consumption. Microgrids can also operate like demand response systems, using utility pricing data to signal to the user when to turn ‘on’ their onsite generation resources to avoid consumption at peak times. This produces a variety of options which include energy availability, supply security, alternative fuel-mix and energy cost reductions.

    Touted as a feasible solution to a wide range of contemporary energy supply issues, such as energy security/supply and climate change mitigation, microgrids are receiving increased attention as a means to power commercial enterprises, including the data centre.

    Of course, microgrids that increase the carbon footprint of the national electrical power generation system are, on the face of it, a negative development. The obvious example that is already used is demand (or frequency) response contracts that can be applied to data centre emergency standby generators.

    In many locations such contracts (like Short Time Operating Reserve (STOR)) offer financial incentives, both through a capacity reservation fee (eg £25k/MW/year) for -100 hours usage per year and a payment at high rate per kWh for the utility draw avoided. However, in all grids the carbon emissions from combustion of diesel fuel-oil on site is higher per kWh of electricity generated than all carbon fuels (other than brown-coal) in power stations, even considering the transmission and distribution losses. For example, in the UK our utility generation is dominated by natural gas, which we combust in combined cycle gas turbines (CCGTs) at a thermal efficiency of ~57% and lose ~7% in station-consumer distribution, so, overall, we achieve a 50% thermal efficiency.

    Comparing this to the 35% of diesel standby generators shows that the net carbon footprint is increased when the STOR contract is enabled. The justification for this is that the STOR is only enabled when the grid is under pressure and having to start up reserve plant in a power station generates more carbon than that emitted by the site generators.

    However, in the past several years in the UK, STOR has rarely been activated, despite repeated annual warnings that we are in danger of rolling blackouts every winter.

    But not all data centres find the STOR proposition attractive. The most commonly held view is that generator power comes with increased risks and the onsite generation is installed to protect the facility from the vagaries of the grid, not to support it. However, there is one limitation from the utilities point of view when it comes to data centres and that is the almost universal data centre feature of partial load.

    Most STOR arrangements ‘island’ the facility rather than run the gensets in parallel with the utility and with the critical load connected. The usual combination of partial load and generator redundancy leads to the generators often running on load at less than 30% capacity.

    This means, for example, that a data centre may have 1MW of installed gensets but when in island-mode only relieve the utility by 300kW of load.

    So, what other onsite options are there that are based on a carbon-reduction basis? There is the option of running bio-fuelled (preferably biogas for low NOX and SOX emissions) generators but the availability is lower than the utility (with genset maintenance taking 2% per year) and thereby needing a fully rated utility connection for standby.

    The maintenance costs are high and the scheme can never approach the economy of utility power while fuel supply and storage on site is always a difficult design and logistical problem. We can then consider onsite solar PV and wind.

    It is easy to discount solar PV to little more than a token percentage of data centre energy demand on the grounds of solar-insolation of 1kW/m2, low cell conversion efficiency of 15-20% and intermittence. Each m2 (ie of roof space) in southern UK will deliver little more than 600kWh/year – equivalent to <70W/m2 over 8,760 hours, perhaps 1% of what is needed for a typical 5kW/cabinet data centre.

    For onsite wind power the data centre clearly needs to be in a rural location (where planning would be granted, no easy passage) and have land to spare for siting turbines. Taking the design to the obvious limit, where the data centre can claim to be 100% renewably powered by wind, produces an interesting problem regarding the utility connection. Firstly, the facility will have to have either a utility connection or a very large energy storage system, or emergency generators with diesel-fuel as the energy store, for when the wind doesn’t blow.

    Alternatively, it can install turbines that can generate as much energy over a full year that the data centre takes to run it and use the grid as the energy storage buffer. Onshore wind turbines can be relied upon to generate 33% of their capacity over a full seasonal year so a 2MW facility would have to install 6MW of turbines. When the wind does not blow sufficiently hard to turn the turbines the facility will draw 2MW from the utility. As the wind strength increases the power draw from the utility will reduce to zero but as the wind strengthens further the turbines will feed the load and generate up to 4MW into the utility.

    There will come a point (often, depending upon site location and exposure) when the wind strength exceeds the turbine’s safe operating speed and the array will brake to a standstill – whereupon the facility will be drawing 2MW from the utility again. So, the data centre operator must pay for an oversized 4MW utility connection to export/sell the excess energy. Although the facility will be net-zero on generation vs utility it will often be consuming utility power that has a carbon-content related to the national fuel-mix. The reader has to consider this fuel profile and reach their own conclusion.

    It is probably worth mentioning that all ‘renewables’ do have an embedded carbon content due to infrastructure – such as concrete dams, submerged biomass in reservoirs, wind-turbine machines and foundations, solar silicon-cells and connection towers, foundations and cables, but what else ‘could’ we do regarding microgrids for data centres that are very low carbon content? Well, in a series of lectures by Rolls-Royce at the IET, a presentation on ‘small’ nuclear reactors pointed to a possible solution.

    Nuclear solution

    Nuclear fission reactors, such as those deployed in our nuclear submarine fleet, are ‘small’ for microgrids (eg 15-20MW), highly available (like a utility connection), not intermittent, safe (we can argue that to the cows come home but morbidity rates for nuclear power are far lower than coal based power), fuelled-for-life (20-25 years) and very low carbon.

    They make an excellent base-load generation source that will enable the maximum possible utilisation of intermittent renewables such as solar, wind and tidal.

    Let’s not pretend that the future is going to be easy with many choices. Recently Stephen Hawking predicted that civilisation as we know it will end in 2600 due to a combination of over-population and energy shortage – and I for one am not willingly going to argue with his level of vision and intellect.

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