A dominant part of operational problems and faults experienced in hydraulic and lubricating systems is due to contamination of the fluid. This complicates maintenance and is definitive a disadvantages of these technologies. Being well documented, both through literature and through practical experience, we have to live with it, and do best out of the situation. Industrially, there do not seem to be a standard method to resolve the problem. Very often, it seems to be a form of religious belief and not scientific knowledge that is the driving force in coping with the problem of fluid contamination.
Hydraulics is normally an area of engineering handled by mechanical engineers. Although control engineers has entered the arena and modern hydraulics is fairly dependent on electronics, the mechanical engineer is still the dominant player. Mechanical filters is the preferred weapon, and the necessary calibre is widely discussed, should a 5m filter or a 2m filter be applied to the particular problem at hand. As doctors, we would probably be sued, if we threated a brain tumour with painkillers or tried to remove a wart with heart surgery. As engineers, we goes free for doing the similar thing.
One should recognise that contamination of hydraulic fluids is more than particles, and that particles is more than mechanical objects. Let us take a different perspective, and use the terminology of a chemical engineer.
A hydraulic or lubricating systems is an endotherm electrolyte where a mixture of fluids transfer energy with metallic surfaces.
The chemical engineer would probably be reflecting over the occurrence of catalytic material and try to analyse the reactive potential of the mixture. At the same time, none of us as mechanical engineers would recognise this as a hydraulic control system connected to the wicket gate of our turbine or the lubricating system of the thrust bearing. However, the above system description could give us a slightly different focus. The chemical engineer would focus on molecular properties and focus on the chemical process that could arise. He or she would probably look at the following parameters in order to obtain chemical stability in the system.
- The composition of the mixture
- The temperature of the mixture
- The amount of catalytic material
In a wider perspective, the access to more catalytic material and changes in the composition of the mixture would be of consequences for the chemical engineers solution to the problem of chemical stability. The system could even be autocatalytic, i.e. that any product from the undergoing chemical reactions will act as catalytic material.
The mixture is normally composed of a well-refined mineral oil, additives, water and air. The hydraulic basis oil is a complex mixture of different hydrocarbon molecules with different properties, among them iso-paraffin’s, naphtha’s and aromatic hydrocarbons. Their ability to react with other atoms and molecules are expressed through the bonds of the carbon atoms. If all carbon atoms are bonded with single bonds the molecule is denoted saturated, and such molecules are normally chemical stable. Examples are found in figure 1, they are all saturated and found in hydraulic and lubricating fluids. On the other hand some types of hydrocarbons have double bonds between the carbon atoms, these are unsaturated and they are less stable than the saturated hydrocarbons. Examples of this kind of hydrocarbons is illustrated in figure 2. Mineral oils are composed of molecules that consist of 20 to 50 carbon atoms and that have an average molecule weight of 400 to 500. These big hydrocarbon molecules normally consist of all the basis types of hydrocarbons with the exception of Olefins that are too unstable and too willing to oxidize.
1 Hydrocarbons in hydraulic and lubricating fluids
Both hydraulic fluids and lubricating fluids based on mineral oils consists of to many different molecules to be analysed on a molecular level. However, it is important to remember that they consists of both saturated and unsaturated hydrocarbons.
2 Reactive Hydrocarbons
It will be difficult to write up exact chemical formulas for the potential chemical reactions in hydraulic and lubricating oils. However, we do know something about what is going on. Oxidation is a process where the hydrocarbons are attacked by oxygen and are broken down to different oxygen rich molecules such as hydroperoxide, alcohols and organic acids. This process can be described in a simplified manner as;
RH + [HEATH] -> R* + H
Where RH represents a hydrocarbon molecule consisting of a reactant (R) and a hydrogen atom (H). If the necessary amount of heath energy is available, will the hydrocarbon molecule be cleaved and a free energy rich radical (R*) and a free hydrogen atom will be created. The energy rich radical (R*) can bond to an oxygen molecule and establish an energy rich peroxide.
R* + O2 -> RO*2
This peroxide can in turn react with the hydrocarbon and a hydroperoxide and create a new energy rich radical.
RO*2 -> ROOH + R*
The reactions above are by no means exact, but they give us an indication about the reactions that may occur in the fluid. Most importantly, they give the chemical engineer the possibility to take qualitative precautions towards obtaining a system in equilibrium and give the system a long life.
- Reduce the amount of reactive material. To initiate the oxidation process, oxygen must be available. Both air and water can supply oxygen, and should be eliminated from the system
- Reduce the amount of catalytic material. Catalytic material, in this context will primarily be copper and iron.
- The oxidation process is endothermal, i.e. heath is necessary. If the operational process is kept below 65 0C, oxidation will only rarely be a problem in modern oils, which is given oxidation inhibitors.
- Add an inhibitor. This is normally done, but it is important not to exceed the recommended temperature if these inhibitors should have the desired effect. It is also important that the oil is free from catalytic material. These inhibitors only slow down the oxidation process, and will eventually not have any effect.
To the benefit of mechanical engineers, the chemical engineers solution to the problem of contamination do not differ to much from that of the mechanical engineer, even if the starting point for the chemical engineer was far more fundamental.
The following factors contribute to the change of petroleum based hydraulic and lubricating fluids;
- The amount of air that can be dissolved depends on pressure and increase with increasing pressure.
- High temperatures, temperatures above 60 0C will accelerate the oxidation process
- Agitation, high turbulence in the flow line will accelerate the oxidation process
- Catalytic action, the presence of metals such as iron and copper will in the presence of water greatly increase oil oxidation
- Sunlight, if the oil is exposed to sunlight a photo-catalytic process will occur and the oxidation process will accelerate.
- Condensed water will contribute to the catalytic action and drastically increase the oxidation process
- Foreign contaminations such as greases, dirt, moisture and paint all contribute to the oxidation process
- Pressure, high pressure increase viscosity and therefore increase the heath production in the system
However, some important aspects is missed. Primarily of tribological nature. Both a hydraulic and a lubricating system produce particles through a number of wear process, and therefore produce catalytic material. Similar to the chemical autocatalytic process described above a spiralling process occur. All parts with relative motion produce particles, and these particles will again produce more particles. Consequently are we both mechanically and chemically dependent on a sufficient mechanical filter so we efficiently can get rid of these particles. The removing of particles, reduce the amount of catalytic material and therefore reduce the catalytic material available for the oxidation process. The oil will consequently increase its life span and the wear inside the system components similarly be reduced. A good filtration system is without doubt a good investment.
The removal of particles, the catalytic material, is only solving one problem. In addition, the amount of reactants and the driving force heath must be controlled, i.e. air, water and oxidation products. It is obvious that some is achieved by careful design considerations and installation of proper filtration, however, the removal of reactants such as oxidation products cannot be achieved by mechanical filtration, as this is contaminants on a molecular level. This can only be achieved by electrostatic cleaning, and one must again move into the world of chemistry.
There is however, one component in most hydraulic and lubricating systems where mechanical engineers, or any other engineer involved in the construction of these system just by pass every considerations made in order to construct a healthy system, and that is the reservoir. It is probably the most neglected component in any hydraulic and lubricating system. In addition it is given tasks which it is not designed to do in any efficient way, and end up as the one component that completely distort the chemical balance of the system. Textbooks formulate the following objectives;
- The reservoir shall act as a buffer for variations in oil consumption, a variation created by the systems working cycle or by thermal expansion of the fluid or both.
- The reservoir shall transmit heath to the surroundings, contributing to a lower working temperature for the system
- The Reservoir shall through the sedimentation of contaminations, contribute to a cleaner fluid entering the system.
Only the first of these tasks is it actually designed to do in any efficient way, and even when performing this one task it contribute to the admittance of reactants as water and air. Any heating engineer could tell us that it is a disaster of a heat exchanger, and any water and sewage engineer will tell us that it has not any change to act as a sedimentation basin with an extensive increase of its volume. Not to mention the frustration of the chemical engineer telling us that we has done everything worse by creating a real electrolyte. We need to move away from the cubic reservoir, and create a new geometry that do not store contamination, but lead them to the filter and other remedies for extraction. Such geometries has been developed successfully.
Renewable technology are on the political agenda as never before, wind, wave and solar power are the new words all politicians’ vocabulary. However, hydropower, which was the foundation of the industrial revolution and as such the historical basis for the modern society, is not mentioned with the same frequency. Hydropower technology is not “sexy” enough. Why is it so? There are most likely more than one answers to this, but I believe that part of the reason is the hydropower industry’s lack of ability to take on technology achievements from other technology areas. The hydropower industry, to put it mildly, is somewhat conservative.
To make an analogy. When the oil and gas industry concluded, that the offshore platform solutions would not be a profitable solution for the development of new oilfields, subsea technology emerged. Still expensive, but far cheaper than the old platform based technology. When the focus in Norway changed from large hydropower plants with tens and sometime hundreds of MW installed, to small hydropower, less the 10 MW, they changed the controller from mechanical to electrical. Everything else stayed the same, just a downscaled version of the large power plants. Not, really at quantum step in evolution.
Overall, the growth of renewable energies will for all technologies, hydropower included, be depended on the electricity price and the investors’ prediction of the obtainable price in the future. The necessary investment pr. produced unit (kWh) will obviously be a factor that will be included in the analysis of the profitability of the investment. This is basic economics, and the need for profit will never change. The profitability, regardless of the type of renewable or other type of energy, is dependent of the following factors.
- Investment pr. produced unit (kWh or MWh)
- Electricity price pr. produced unit (kWh or MWh), present and future
- Operational and maintenance cost
In addition, there are risk factors that create uncertainty to the analysis. How much will it rain, will it be dry years or will it be wet years. How many sunny days, etc. Every source of energy will have its uncertainties, but here focus will be on hydropower. Taxes and electricity prices are out of the developer’s control. However, the developer will always be able to find predictions and estimates are available. However, The Developer can influence the size of the investment and the operational and maintenance costs, active costs or passive costs.
- Civil engineering works, i.e. dams, pipes, gates, powerhouse etc.
- Operational and maintenance cost
- Grid connection
- Electro/mechanical equipment, i.e. turbines, generators and accompanying equipment
The active, cost is the only ones that will influence the future earnings. The passive is necessary, but will not, except in very special cases, influence the future earnings. However, there is an obvious link between the selection of electro/mechanical equipment and the maintenance and operational costs. The developer should always minimize the passive costs, and rather spend money on the active elements. Of course, the economics still have to add up. As there is requirements set by the authorities, there is not a total freedom in specifying civil works. Remarkable enough, many developers, even professional developers, do not comply with this somewhat elementary economical principle.
Efficiency of a turbine will always deteriorate over the lifetime of the turbine due to wear, corrosion, cavitation or other causes, of which, some can be eliminated and some, which effect can be delayed. As it affect future earnings, careful considerations is necessary when selecting technology and configuration of a power plant. In addition, the operational plan of the power plant will also effect the selection of technology.
- Investment costs, both infrastructure and equipment
- Effect on future earnings
- Refurbishing costs, i.e. repatriated to its original condition
- Maintenance cost
In addition to the traditional turbines Pelton, Francis and Kaplan operating at constant speed, there is available a number of other possible selections which can be more suitable and more profitable for a specific project depending on the project’s characteristics. Including, but not restricted to the following technologies;
- Variable speed operation of medium and low head turbines, and even high head turbines with long penstocks
- Direct coupled generator
- Belt driven high speed generator
- Archimedes screw
- VHL turbine
- Low Head turbines with integrated generator
- Submerged generators for open pit power stations for both synchronous and variable speed operation
The main bottleneck for the implementation of these “innovations” in both new power plants and power plants under modernisation is mainly the same conservatism of the hydropower business. However, also the research establishment is in my experience to blame. The research establishment is too much focusing on the optimization of the traditional turbine types, and do not know enough of the technology development in other areas of technology to contribute to the development of new solutions.
At a large number of hydropower plants there is enforced restrictions on biological flow, i.e. the dam must release a certain amount of water to the original river. This amount of water is seldom utilised for power purposes and represent a loss. Some exceptions is known, the Leiro power station in Eidfjord municipality in Hordaland county (Norway) utilises the water released from the Sysen dam in order to maintain the minimum water flow at Vøringsfossen – one of Norway´s main tourist attractions. A minimum of 12 m3/sec of water is released from the dam in the period 1 June until 15 September. The plant was set in operation during the summer 2011, have an installed power of 5.0 MW and an estimated production of 8.0 GWh. The Sysen dam has a difference between (HRL) and (LRL) of 66 m. The Dam is 1160 meter wide and 81 meter high. A production of 8.0 GWh, is a considerable gain, that otherwise would have been lost.
The large difference in head due to the large difference between Highest Regulated water level (HRL) and the Lowest Regulated water level (LRL) is a main concern for any turbine designer, as this will effect both the turbine efficiency and the cavitation performance of the turbine. It is the relative change of head, and not the absolute change in meters that is of importance in this context. The higher specific speed the turbine have, the larger effect, i.e. Propeller type of turbines suffer more from head variations than Francis type of turbines, deductible directly from the respective efficiency hill diagrams.
To some extent, one can overcome this problem by introducing variable speed operation. Although, permanent magnet generators are of limited availability, generally are more expensive than normal synchronous generators and need power electronics in order to maintain the correct grid frequency, in many cases this still will be a profitable solution. Alternatively, a double feed generator can be used, although this limit the speed variation possible to achieve. The main concern will be the distance to a connecting grid, as long connecting cables can kill any feasible project. A was project proposed for a dam with biological flow in the South of Norway, where the Head variation was 4 m, varying from 12.8 m to16.8 m, i.e. a change of approximately 23 %. In addition, the biological flow restriction was set to 2.0 m3/sec for 7 months of the year, while it for the remaining 5 months was 3.0 m3/sec. The project had an estimated production of approximately 2.4 GWh, and would have a payback time of 10 years based on an assumption of future electricity prizes. The project would have given a positive contribution to cash flow from the first year of operation. The use of a variable speed propeller turbine was essential to the project. A synchronous speed turbine would not be suitable under such operating conditions. As the outlet from the power station was at the same point as the release of biological flow, no violation of the biological flow restrictions was foreseen. The installation would have given a good documentation of the released biological flow, something that was not possible by releasing flow through the floodgates. Unfortunately, the power company made other priorities.
There are many existing hydropower projects around the world where a similar project, utilising the biological flow released from dams, can be introduced. Giving the operator increased profit and give the licensing authority’s better control with how the biological flow restrictions is practised.
The application of variable-speed generation schemes to hydroelectric power plants offers a series of advantages, based essentially on the greater flexibility of the turbine operation in situations where the flow or the head deviate substantially from their nominal values. Specifically, the following aspects may be emphasized:
- In general, for variations in head and flow, the variable speed option would be more advantageous.
- Running the turbine at a variable speed avoids cavitation and draft tube oscillations.
- In hydro plants with reservoir, the operating range of head variations can be increased, thus reducing the need for flooded areas.
- In the run-of-the-river hydro plants, the continuity of operation may be increased because of the higher range of allowable flows in the turbine.
- A simulation performed on a run-of-the-river small hydro plant confirms that significant gains in generated energy may be obtained.
|Variable Speed Francis and Propeller turbines|
- Higher Production (kWh)
- Increased income from the Power Plant
- Simple design, less operational problems, cheaper maintenance and less loss of production due to failures
- High efficiency over the whole operational area
At first let me state that this by no means will be a scientific paper, this paper express nothing else then some thoughts about the future of Small Hydro Power, and Hydro Power in general, that solely is on the account of the author. Further, more questions will be asked then answers given.
Figure 1 shows the traditional layout of a Low Head Hydro Power Plant, and it is mainly the same whether the installed power is 100 MW or 100 kW.
Hydro Power has a long tradition, centuries back, and probably the discovery of electricity changed the business more then any other happening through the history. Electricity enabled the transmission of power over relative long distances, with out significant losses compared to the old mechanical transmission system. At the same time it introduced the large power schemes which produced more energy then the local community could consume. However, nothing else has changed much. Oftedal Power Station (3MW) was opened in southern Norway in 2006, and went into the history as another Hydro Power Station with a architectural prize for design. Through history one could sometimes believe that achieving such prizes were the major force for any Hydro Power Development. Engineering pride, instead of engineering creativity has been the case much too often.
Returning to Figure 1 it is easily recognized that all components that together make up the power station, dam, generator, turbine runner, turbine guide vanes, spill gates, other gates, is therefore a purpose. An approach to reforming the structure of a Small Hydro Power plant would obviously be to look on the functionality of each of these components in a framework of a power station that is not dominating the grid, but only supply a fraction of the power that supply the grid. There must be a difference between the specification to be complied with between a power station that deliver 300 MW to the grid, and the power station that supply 1 MW to the grid.
What is the purpose of a Dam in a low head power station placed in or by a river, as illustrated in Figure 3? The answer is rather obvious. It is going to focus the water towards the turbines rather then allowing it to flow down the river. If the topology allows it can also be used to gain Head, and therefore increase the energy output of the power plant. Which factors decide on the dam’s construction, again there is an obvious answer to that? The local geology and the water pressure acting on it.
What if there is coming more water down the river then the turbines can swallow? Well there is an obvious answer to that too, the water level behind the dam will increase and eventually flow over the crown of the dam. To have a controlled flow through the dam, that will not harm it spill gates are installed. In large dams where the hydrostatic pressure on this gates are substantial hydraulic operated sector gates made of high quality steel is often installed. On a small dam where the hydrostatic forces are smaller, this can not be necessary. An American company has come up with a smart solution, shown in Figure 4, which combines the dam and spill gates where it is taken benefit from the low hydrostatic pressure in the selection of materials and actuating system. The gates can be placed directly on a concrete foundation (step) an will form the actual dam itself. Compared to the balloons we use in Poland, this system is active and more wear resistant than the balloon is. The system is patented, and shows engineering creativity.
The wicket gate or guide vanes of a turbine, Figure 6, serve mainly to control the flow into the turbine runner and through that the output of the turbine. In High Head power plants where long penstocks occur they must be carefully controlled so no dangerous pressure transients occur during operation or close down on the turbine. In Low Head power plants this is not a problem and the wicket gate are mainly used to control the turbine output in accordance with the demand in the grid. In a small turbine that produce 1 MW, there normally is not a need to control the output because the produced power at all times will be lower then the minimum consumption in the grid it is connected to. The wicket gate could therefore easily be omitted without any consequences. However, there could be environmental constrictions that make controlling the flow (not output) and in these cases the wicket gate still will be needed. The above argumentation seems quite logical, at least to the author, but still most new Small Hydro Power Plants are equipped with conventional wicket gates, in fact the author only know one manufacturer in White Russia who offers Kaplan turbines without this controlling device. I might very well be wrong on this, but that only underline my continuously need for learning.
Going around looking at Small hydro Power plants, the author have found that there a substantial amount of them that have quite long open supply channels. They are not only long but they are very nicely made, some of them even have sidewalls made of neatly laid natural stone. The association to Roman aqueducts feels quite in place when admiring the stone work. It is like the owner wants to leave a monument behind, a monument that can be admired a thousand years later like we to day admire the Roman aqueducts and Colosseum in Rome.
- However I see some problems.
- They are expensive
- They collect a lot of trash
- They must be frequently maintained to keep a low flow resistance
- They are a hazard to the public
There is an obvious alternative, dig down a plastic pipe. There have to be some digging to make the channel anyway. This way most of the disadvantages listed above will disappear. Not only that the impact on the surrounding nature will be less obvious, and one could even create a park for people to enjoy them self.
At last let us return to the turbine. Since Edison the Hydro Power business have taken constant speed operation as a law of nature. In relation to classical technology this was most certainly through, the generator had to operate at constant speed in order to maintain a constant frequency. Today, with the developments we have had in electronics this is by far a law any more. Technically there is no necessity for the turbine to operate at constant speed, electronics fix everything before the energy is supplied to the grid.