Cement industry consultant Xavier d’Hubert has recently turned his attention to the use of micro-grids, localised electrical supply networks that work to augment or replace national or regional grids. Here he discusses the possible application of microgrids to the cement sector...
GC: Please could you define the term microgrid?
Xavier d’Hubert (XDH): I think there are as many definitions as players in this field but what is commonly accepted is that a microgrid is an interconnected system of decentralised electricity production with some form of energy storage, most commonly batteries. The microgrid can either be connected to the main grid, work as an island system or both.
GC: How do connected and island systems differ?
XDH: Connected microgrids are linked to the main electrical supply grid in a given locality. They are seen mainly in developed markets, where they supplement steady national supply lines.
Connected microgrids can be connected in front of the electricity meter, i.e. at a substation. In this case the purpose of the microgrid’s storage capabilities is to assist in regulating against fluctuations in frequency and voltage.
Connected microgrids can also be installed behind the meter. This is commonly used where the microgrid supports a single end user, be it an industrial facility or a large commercial building. Behind the meter microgrids will usually also generate their own electricity and have an energy storage facility with a duration of 2-6hr.
By contrast, island mode microgrids are, as the name suggests, not connected to the main electricity grid at all. They are used in remote areas, for example for touristic developments or mining operations where the cost of supplying power is prohibitive. These obviously have integral power generation and storage capabilities. Some microgrids have the ability to switch between connected operation and island mode.
GC: What forms of energy storage can be used?
XDH: The main type of storage is batteries, which are becoming increasingly flexible. Aside from short-term storage, which is already taken care of by lithium ion batteries, there are a number of developments. CMI Energy storage, part of the CMI Group that I work with, is building a microgrid in order to test various batteries. It is closely following the development of flow batteries for long-term storage. In these systems the electrolyte is outside of the battery itself, a bit like a fuel cell.
The batteries can discharge at different rates depending on the requirement. It could be used to discharge at 100% over 2hr, 50% over 4hr or 25% over 8hr. A similar development is the hybrid flow battery, where one side of the battery is a steel plate where ions are exchanged. The other side is a liquid solution of iron and zinc (or others), both of which are abundant.
These flow and hybrid batteries allow for the decoupling between power and energy, with their external electrolyte tanks that can be sized independently of the numbers of stacks.
Oils and molten salts are also used in the concentrated solar power (CSP) plant sector, especially for power / tower installations that can generate 50 - 200MWe. They have very high heat capacities and store thermal energy for 8 - 16hr. They help turn solar power into a 24hr opportunity.
Flywheels can also be used. Initially they were used to help ‘boost’ trains when leaving stations over periods of seconds or a few minutes. They can also be used to help frequency adjustments over short time frames. They are becoming more applicable all of the time. One manufacturer now even offers a flywheel that can store energy for up to 4hr. This involves a metal disk (size and weight according to the energy storage capacity), which rotates at up to 10,000rpm in a vacuum.
GC: What are the potential benefits of a microgrid for a cement plant user?
XDH: There are a lot of things a cement plant can do to optimise its power use and microgrids offer a number of advantages. The main objective is to reduce the electricity bill.
Firstly, load-profiling can be used to observe electricity consumption patterns to identify and eliminate peak demand periods, much like a domestic smart meter. This information would allow producers to correlate consumption with facility activities and production in real time to forecast and reduce energy demand. Similarly, gathering and reviewing power quality information can help identify power system anomalies and provide calculations like the cost of a possible power outage.
Once the above information has been gathered and acted upon, demand management systems can be used to reduce overall energy demand through load shedding and peak shaving strategies, most simply by buying power cheaply at night and storing it for use in the day. It helps reduce demand changes and manage real-time power purchases. This benefits not only the cement plant but also the grid due to reduced demand peaks and troughs. This raises the efficiency of the system and helps to avoid power outages.
Aside from cost reduction through peak-shifting, microgrids could help to iron out many of the electrical supply problems that might face a cement plant. These include unfavourable harmonics, voltage excursions and power outages. Microgrids protect generators from dangerous and damaging overloads, maintain critical loads during outages and / or allow for a more controlled power down procedure. This would stave off some of the costs associated with damaged equipment and downtime.
GC: Are there already examples of this type of system in the cement sector?
XDH: Some would say that any cement plant that uses a diesel or gas gen-set or captive thermal power plant, and there are many examples across Central America and Africa, has a ‘microgrid.’ This is correct in a way, but most often there is no storage capability. Storage is the key to so many of the advantages that microgrids offer.
There are already moves towards these kinds of microgrids in the cement sector. A cement plant in California installed a wind farm to supply its own electricity in 2011. Initially it was only a captive facility in that it only supplied the plant. The cement producer was not allowed to sell excess power to the grid.
Now, as thermal power plants are being closed in California, there is increased demand for renewable power sources state-wide. The rules have been relaxed to that the plant could harvest energy, store it and then distribute it to the grid during the peak demand in the evening. Such a cement producer could now be looking at installing batteries to shift the peak. The flow batteries I mentioned earlier have a capacity of up to 800kWhr each (in two 40ft long containers). They are now becoming commercial and will be able to answer a lot of the questions regarding energy storage for peak-shifting with renewable energy sources.
In another example from elsewhere, there are penalties in Ontario, Canada for any user that takes even an extra 1MW during the five highest peak demand periods of the year. This is known as the Global Adjustment mitigration strategy. Most of the cement plants there will now shut down most of their equipment, principally mills, when they see a peak coming. With a fine in the magnitude of US$400,000/MW, it is expensive to overshoot. A lot of these types of producers would be very interested to bring control of their electrical peaks and troughs in-house. Microgrids provide a way to do this that lowers consumption and cost.
GC: Do microgrids have disadvantages?
XDH: Microgrids are quite a lot more complex in real life than they are on paper. This can be off-putting to potential users, even when there are significant benefits on offer. There are still not many standard pieces of equipment that are routinely used and no OEM dealing in such plants. The user has to work everything out, convert AC/DC (and sometimes back again), and consider all of the flows and possible situations that might occur. CMI is acquiring that first-hand knowledge from its MiRIS microgrid project.
GC: What are the chances of widespread uptake of cement plant microgrids in the next decade?
XDH: This question can be split in two. Firstly, grinding plants are a lot smaller than integrated plants, often requiring just a tenth of the electrical load. Their smaller scale, plus the fact that new grinding plants are coming online in regions where decentralised power is already a common solution, mean that microgrids for grinding plants are fairly likely in the medium term. Solar power would be a good option in many regions, particularly across parts of Africa, where the existing power supplies are poor, polluting, and/or expensive (diesel generators). I can tell you that many cement companies I meet are looking at this issue very closely.
For integrated plants, microgrids remain very useful, but they are unlikely to power a complete plant. Microgrids would instead be used to supplement existing power supplies. The benefits of the microgrid in this case have been outlined earlier.
In both cases, I think it’s unlikely that cement producers will be the main investors in microgrids. Instead they will want to shift the risk to a supplier and then buy the energy from the supplier under a supply contract.
GC: Indeed - This is how some cases work already, for example at Hanson’s Ketton plant, which has an adjoining solar farm.
GC: How do you see the future of the grid / microgrids? Will we really end up with a decentralised system that smooths out all peaks and troughs?
XDH: Microgrids have features that eliminate some of the big disadvantages of large power networks. Due to this and the falling cost of battery storage and photovoltaic (PV) cells, microgrids will become more popular for a wide range of sectors, including the cement sector. In many cases they will operate in tandem with the main grid and, eventually, they will support and feed off each other too. The web-like systems that will result from this process are likely to vary from place to place but they will all be efficient systems that have developed by ongoing iteration to find low CO2 and low cost solutions.
It feels like the battery technology is now at a ‘take-off’ point like PV cells were 10 years ago. If batteries follow a development trajectory even half as steep as PV has, they will drastically alter the way we all use and store electrical energy, not just cement plants.
GC: Thank you for your time today Xavier.
XDH: You are very welcome.