A Simplified Guide to Best Generation Mix for Future
By : Eng. M. Lakshitha Weerasinghe
Electricity accounts for less than 12% of the total annual energy used in the country. Yet it draws much bigger attention than the remaining 88%, possibly due to high commercial attraction and business opportunities the sector offers. On average, Sri Lanka produces over 40% of its annual electrical energy requirement from renewable sources, a very high proportion among fully electrified nationsby any world standard. In certain years such as 2013, this figure had gone past 60%.Yet, electricity sector is criticized for not doing enough and absorbing more renewables.
Deciding the countries optimum electricity generation mix is a highly specialized technical subject. Such mix must be decided so as to balance out key objectives, namely, security of supply, reliability of supply, affordability of supply and environment friendliness/sustainability. If onetries to maximize economics which has a direct impact on affordability, that could violate environment friendliness and security.while if another tries to strive more of environmental friendliness, that could violate affordability and reliability.
This article is aimed at giving a simplified, non-technical explanationas to what would be the best energy/fuel mix for future Sri Lanka that not only maintainthe already high renewable share of electricity but also maintains reliability and affordability of electricity supply. In order to explain this to a non-expert readership, analogies with other sectors and simplified examples are used.
Generation mix and the transport mix, a simple comparison.
The transport sector of any country must have a mix of transportation modes, ranging from mass transport such as trains and busses, to ecofriendly bicycles. Just because rail transport is the most economical under certain circumstances, we cannot only have rail transport to provide all our transport needs. Just because bicycles are the most environmentally friendly mode and is economical, we cannot meet all our transport requirements only from bicycles. A transport sector needs a mix of mass transport modes such as trains, busses to take people to places where trains cannot go, cars to take people when and where busses are not available, motor bicycles and three wheelers to take people who would use them for specific needs and to suit their purse and bicycles to supplement all the above.
A properly planned power system is no different. A power systemtoo requires a mix of generating technologies to cater to demand.
Meeting Supply and Demand of Electricity in Real Time, the Complicated Balancing Act.
Meeting demand for electricity is much more difficult than meeting transport needs as, (unlike transport where commuters can wait to catch the train or bus), supply and demand for electricity needs to be matched Mega Watt (MW) to MW in real time. Unless supply (generation) of electricity is equal to demand, an electricity grid collapse. Thus, an electricity grid must have a carefully planned mix of generating technologies that can readily follow consumer demand to cater to different patterns of demand for electricity during different times of the day, which is dynamically changing.
The picture shown here (which is copied from a magazine of the Swedish Grid Operator) shows how System Operators at National Grid Control Centers world over (Sri Lankan equivalent is the System Control Centre at Sri Jayewardenepura) balances the customer demand on the left side of the scale using controllable generators at his command at the right side.
In traditional power systems, all the changes have been taking place on the left pan of the balancescale where customers change their electricity needs second to second. System Operators (the gentlemen shown at the middle) change the output of conventional generating plants on the pan to the right to adjust the customer demand so that supply is always equal to demand. In such conventional power systems, no grid connected power plant is allowed to start, stop, increase, decrease their output without a dispatch command from system operator.
However, modern power systems have given system operators a new challenge as, now they not only have to deal with the changing demand on the left pan but also the changing generation from “Non-Conventional Renewable Energy” - NCRE,(also called “Other Renewable Energy” - ORE) based generating plants such as wind and solar on the right side of the balance scale due to changesin wind speed and solar irradiation. To make things worse, such plants cannot be controlled by system operators either and hence would generate when they have the wind/sun and stop/reduce when they do not have the resource, making this balancing act increasingly difficult.
For a typical day in Sri Lanka, the customer demand graphically shown on the left panvary from about 1,100 MW (occurring at about 2am in the morning) to 2,600 MW during the night peak(occurring at about 7pm). Additionally, the system operators also now have over 800MW of installed NCRE based generating capacity on the right side, that too changes depending on resource availability. Thus, in order to do this balancing act in islanded power systems like that of Sri Lanka, we need installed firm generating capacity. It is such firm capacities that the system operators need to use to balance out not only the customer demand but also varying generation from NCRE plants.
Developing Firm Generating Capacity to Balance Non- Firm Capacity.
Now we come a step closer to explain how to decide the optimum generation mix, the firm and non-firm. Firm generating plants are the plants that are available any time the system operators want them to operate. When in operation they give a stable firm output as decided by the system operator. Examples of firm generating technologies are storage hydro plants of CEB (such as Victoria, Kothmale, Laxapana), coal steam plants, Gas turbines, combined cycle plants, and diesel engine-based plants that are in our system or Liquefied Natural Gas (LNG) based power plants to be added in the future.
All NCRE based plants such as wind and solar gives non-firm, varying output that changes minute to minute of varying degrees when a cloud passes overhead or with wind speed changes. They cannot be commanded by the system operators.
Thus, whenever the share of NCRE capacity in the system goes up, we cannot give way to such plants and stop developing firm capacity but also need to keep on developing firm capacity too to balance out output from non-firm plants and to bring them in when wind and solar are not available.
One may view firm capacity as the foundation and walls of a power system on which other non-firm capacities too can be added. More on lighter terms, the firm capacity is like the main meal of a person while the non-firm, non-conventional renewable capacities like solar and wind are like the starters and the dessert! There is no way the starters and dessert could substitute a main meal though they can very well supplement the main meal.
Thus, a power system must consist of a correct mix of installed firm capacity technologies, supplemented by NCRE plants. Whenever NCRE plants such as solar start generating power, the firm generating plants in the system will reduce their output to make way for electricity coming from non-firm plants and operate either on part load or on standby. (Such part loading of firm capacity generators too adds an additional operating cost to the system and hence NCRE plants adds a hidden operational cost, further explanation of which is beyond this simple article).
The Unique, Isolated, Islanded, Small Power System of ours!
The task performed by the man in the ‘picture’ is a highly specialized and skilled one. It is more so when he is operating an isolated, small power system like in Sri Lanka having a significant share of its firm capacity too based on storage hydro power plants. (Storage hydro plants are always operated giving priority to needs of other stakeholders such as agriculture and water supply authoritieswith whom the water resource is shared. However, how a hydro system is planned and operated is another highly complicated subject beyond this simple article).
Often, those who are not having any understanding, experience or expert knowledge about system operations or how islanded small power systems such as ours work tries to criticize CEB by citing examples of European countries with large interconnected grids. They show examples of days and times where certain countries like Spain and Germany have produced their total or sizeable requirement of electricity from wind and solar and suggest Sri Lanka too should strive to do what such countries(occasionally)do. But they propose doing so not occasionally but 100% of the time and becoming a fully renewable grid. But if they can do so, why not us? Let us try to understand.
Though you may not know, even in Sri Lanka, there are many a times where certain areas of the country are supplied 100% with NCRE based generation such as mini hydro and solar power. For example, on sunny days during wet season, certain grid substations like Ratnapura and Balangoda having a large number of embedded mini-hydro plants generate the entire demand of customers connected to such grid substations using mini hydro and solar and even provide the balance to the national grid. This is not much different to what is happening in certain European countries like Germany and Spain, which are connected to a very large common European grid. Just as Ratnapura and Balangoda are interconnected to a large grid and hence the firm generating plants in operation elsewhere can balance out the local (embedded) generation in such areas, excesssolar and wind generation from Spain and Germany could be balanced out by the installed firm capacities in other countries interconnected in the common European grid.
Thus, to a layman, a kilo Watt of generation from a Solar plant may be similar to a kilo Watt from a thermal plant. But in reality, they are different, and more so if you are in an isolated grid.
Mechanical Inertia and Synchronous Power Grids, the Lesser Known Facts.
Another subject that is not understood at all by non-power system engineers and environmental enthusiasts is the need to have what is termed “mechanical inertia” to operate a synchronous power grid. As this is a highly theoretical and specialized subject area, a simple explanation that could be understood with basic science knowledge is given below to explain what conventional firm capacity generators give in the form of inertia (in addition to electricity) that solar and wind plants are not capable of.
We explained earlier that demand for electricity must be matched by equal generation in real time. However, in real time, this balancing act is not done by changing generation by the system operator in the picture, but by utilizing stored kinetic energy on generators and turbines operating in the system at the time. The amount of kinetic energy stored in a power system at a given time is determined by the mechanical inertia of the system. All conventional generating technologies give mechanical inertia to the system, thus assisting the system operators to supply demand for electricity in real time until he intervenes and adjust output of generators on theright hand side of the balance scale in the picture. But NCRE technologies such as Solar and wind do not add any mechanical inertia to a system. Thus, when there is a large number of solar and wind plants operated at a time, if we back down conventional firm capacity plants to make way for such wind and solar plants, the amount of mechanical inertia in a system goes down, making such systems unstable and weak
The Future Generating Mix- the balancing act.
There are certain technical solutions such as batteries with the aid of which we can reduce some of the drawbacks mentioned before and thereby add more and more Wind and Solar. But are we ready to afford such costs and for what reason? We must understand what is our objective.
A mere one-rupee increase to the average cost of a unit of electricity causes total costs of CEB to go up by an additional 16 billion LKR an year (as approximately 16 billion units were generated in 2019). Thus, increase in generation costs can impact the national economy more than any other single factor.
As explained earlier, we have to balance out economics and reliability with environment friendliness. We have already achieved environmental friendliness, not only in the electricity sector but as a country as a whole.
For example, if you take whole of Sri Lanka, per capita CO2 emissions of Sri Lanka (from all sectors) is 0.99 tons of CO2 equivalent per year per person, which is already far below the global average of 4.35 tons per person. Sri Lanka’s emissions is very low even by regional standards as India emits 1.57 per person, Indonesia 1.74, Malaysia 6.93, Thailand 3.55 and USA 14.95. Total CO2 emissions of Sri Lanka in 2016 is 20.9 million tons per year and is only 0.06% of the total world emissions. Even in 2039, this figure is to be around 24 million tons and hence Sri Lanka is already a very good global citizen and is expected to remain so.
However, the same cannot be said about economics. Our costs of generation are already high due to incorrect generation mix. Thus, a “corrective action” is required to restore economies of supply and reliability of supply as environment friendliness is always at a sound level.
However, if you see the adjoining graph, you can see that the amount of Non-Conventional Renewable Energy -NCRE (also termed as Other Renewable Energy -ORE) that we have planned to be absorbed to system (with all the difficulties mentioned before). When compared to the total NCRE (ORE) capacity added during the past 20 years, a fivefold increase is expected to be added in the next 20 years.
Thus, CEB has already facilitated via its plans the absorption of a very high percentage of renewable energy. CEB is also pursuing a novel ''semi-dispatchable" operating strategy with the 100MW Mannar wind plant to be commissioned in 2020. Once tested and mastered, it is expected that more wind resources could be added to the system using such operating strategy. Thus, with experience about our own national grid, CEB is continuously pursuing on how to absorb more intermittent resources to grid. Howeverin order to absorb such non-firm resource based generation; we also need to develop firmcapacityincluding thermal base load plants such as coal andLNG andpumped storage hydro plants planned for future.
CEB has already planned to introduce Liquefied Natural Gas (LNG) based generation to Sri Lanka. LNG is regarded as a much cleaner fuel among firm capacity generating technologies. With the introduction of LNG, Sri Lanka expects to generate closer to 60% of its energy demand from cleaner fuels by 2030. This share could be increased up to 70% (with out of merit order operations and hence at an additional cost), if it so require.
CEB’s long term plans are prepared so as to build the most economical firm capacity base and to integrate highest amount of NCRE technologies possible. Such shares are determined after Renewable Integration Studies done with expert assistance. Such planned renewable absorption could be developed/realized with the assistance of other agencies such as Sustainable Energy Authority (SEA), who has more responsibility and a role to promote development of NCRE resources than CEB.
Costs of renewable energy based technologies such as wind and solar is coming down in the world market. However, the actual generating costs from NCRE plants such as wind and solar in Sri Lanka are still comparativelyhigh. Thus, in order to bring home the advantageous of such low price trends in the outside world, a competitive tendering system for NCRE technologies is mandatory.
This article was written in a simple way to explain with reasons why future generation mix need to be carefully planned by those having expertise on such subject. However, to implement such plans, the support of all is required as what was lacking in the past is not plans but implementation of such plans.
Eng. M. Lakshitha Weerasinghe
Deputy General Manager Transmission & Generation Planning
Ceylon Electricity Board
Eng. Lakshitha Weerasinghe has graduated in 1994 from University of Moratuwa with a BSc degree in Engineering and holds a Master of Philosophy degree from University of Peradeniya. He counts over 25 years of experience as an Electrical Engineer, which includes two years in the private sector and 23 years in the Ceylon Electricity Board. He commenced his CEB career in the Generation division, followed by 8 ½ years at the System Control Center, five of which as the Chief Engineer System Operations. Presently, he is holding the post of Deputy General Manager Transmission & Generation Planning. Lakshitha was also a former council member of IESL having served six years in the council and also a former editor of Sri Lanka Engineering News. He held the post of SLEN editor for eight years.