ENERGY STORAGE AND GENERATION
Solar PV
DC AC AC
Wind Turbine
AC AC AC Bus
Battery Storage
DC
a cost of £4.2 m, which provide monthly savings of up to £300,000 on energy bills, and cut Hull University Teaching Hospitals NHS Trust’s carbon footprint, while reducing the demand on the energy grid. Incorporating battery energy storage alongside renewable sources makes it possible to further reduce operational energy costs, while also providing increased resilience with back-up energy storage.
AC Loads Figure 2: Microgrid topology.
mains power provided throughout our buildings consists of alternating current (AC). To convert this power from DC to AC, or vice-versa, it is necessary to use an inverter to feed into a centralised point (AC bus as shown in Figure 2), which can then supply final loads. A microgrid has different protection philosophies and techniques to conventional distribution systems, particularly during islanded operation. The level of fault current capacity available in a microgrid will be drastically lower than in interconnected systems. Further to this, due to a higher possibility of system transients and emergency response requirements, the range for voltage and frequency excursion during islanded operation can be much wider than typically considered permissible. Consequently, a microgrid needs to set voltage and frequency- based voltage protection schemes differently. With this in mind, the existing protective devices, for example circuit breakers, should be evaluated for their suitability to avoid unwanted tripping, and replaced with devices better suited to microgrids. This point is hugely
important for the integration of microgrids within critical infrastructure. The risk of potentially creating additional weakness in a distribution system could lead to disastrous power failure, and subsequent losses – including both human and financial. For these reasons, regulatory standards, such as G99, have been introduced to account for this
What are the challenges? Among the greatest challenges and possible limitations to widespread microgrid incorporation is the initial cost to design and install these systems. Solar PV, for example, can cost upwards of £1000 per m2
,6 and battery storage
systems that use lead-acid and lithium batteries are now commercially available in Europe for about £900 per kilowatt (kW).7
Nevertheless, these technologies
can significantly reduce operational energy costs over their lifecycle, and provide reasonable payback periods. For example, Castle Hill Hospital, Cottingham has recently installed 11,000 solar panels at
Backup Generator
G
Main Grid
A lack of space Space is also somewhat problematic when it comes to healthcare sites and siting of renewable sources and battery energy storage. For instance, the installation of a solar PV array such as the one at Castle Hill requires vast expanses of open space, which is rarely available in more urban healthcare sites. On the other hand, more effective use of rooftop areas, and allocation of space to site these technologies, are necessary to ensure more widespread implementation. Planning during the early stages of consultation and design of extension works or new-builds can ensure that consideration is made for allocating sufficient space to accommodate these technologies. For instance, in France specific legislation on this has been approved, and will see car parks with over 80 spaces require the installation of a solar array.
while tower-mounted wind
turbines (100-2000 kW) can cost upwards of £700 per kW,6
Looking forward Microgrids are still in their infancy, and will likely see more research, and become increasingly widespread, as our cities and energy grids become more ‘connected and intelligent’ to achieve Net Zero targets. As our cities and energy grid become ‘smarter’, we will see greater use of information communication technology (ICT) to ensure flexible, reliable, and resilient delivery of energy and related services. Considering that many healthcare sites have an internet protocol (IP)-based communication network and building energy management system (BEMS), they are already well placed to be incorporated into the ‘smart grid’. In addition, battery energy storage is expected to undergo significant development and advances in the future, especially given that, currently, the raw materials used in lithium-ion batteries are costly and difficult to extract, while the material is also costly to manufacture and recycle. Recent experiments8
carried out on sulphur- Figure 3: Solar PV now increasingly features in car parks, including on hospital sites. 42 Health Estate Journal February 2023
sodium-based batteries to improve the reactivity of sulphur, and the reversibility of reactions between the two elements, have exhibited much higher capacity and life in comparison with lithium-ion batteries. Although this is at an early stage of research, the results are promising, and may eventually offer a cheaper and more environmentally-friendly alternative for battery energy storage.
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