Open cycle OTEC

Following the recent posts, we are going to follow our research with the OTEC (Ocean Thermal Energy Conversion) devices.

Last post, Mikel wrote about the closed cycle OTEC. Well, the open cycle process relies on the same thermodynamic theory: A fluid that evaporates makes a turbine move generating electricity, using later cold water to condense the vapor. But unlike in the closed cycle, the substance that is vaporized in the open cycle is seawater, to be exact, surface seawater.

But of course, if pure H20 vaporizes at 101,3 kPa at a temperature of 100ºC, you might be wondering: how can the surface seawater, being at a temperature of around 20-25ºC vaporize? Well, the answer can be found in the Pressure-Temperature diagram of the water.

The fact is, that the chamber where the water is vaporized is a low pressure chamber. It has a vacuum pump that regulates the pressure so that the phase change can take place at those temperatures.

We can observe in the diagram, that if the pressure is lowered, the boiling point of water is consequently also lowered. Therefore, as the chamber is a low-pressurized, we can make the warm seawater change phase turning in to vapor.

This vapor is then used to make the turbine turn. But, what happens with it later? Well, this is where the second part of the process turns in to action! The cold water that is brought up from deep down under acts as a refrigerant , and turns the vapor back into liquid again. But of course, this is basically pure water, because as it vaporized, it left all the salt and mineral behind. This way, we obtain desalinized water! This water can be useful as drinking water or desalinized water.

 

Information was mostly gathered from wikipedia.

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The vision of Jules Verne

Everybody knows that the famous French novelist Jules Verne was able to predict a large number of inventions and discoveries unknown in his time. In his book Twenty Thousand Leagues Under the Sea Jules Verne predicted the Ocean Thermal Energy conversion, when a character of the novel, Captain Nemo, alluded to the possibility of taking advantage of the thermal energy that is in the oceans. And this is not only a literary curiosity. The engineer Georges Claude built for the first time a prototype plant of that technology in Cuba in 1930, and he took the idea from the phrase of Captain Nemo.

The system to exploit Ocean Thermal Energy, as Junkal wrote in her last post, is similar to a conventional thermal power plant.

There are different methods for using Ocean Thermal Energy Conversion (OTEC) to generate electric energy. These methods are based on “Rankine cicle”, a thermodynamic cycle in which a liquid is evaporated by heat, and then the vapor activates a turbine that can be used to obtain electric energy.  However, there are several cycle systems for it: closed cycle, open cycle and hybrid cycle.

In this post I explain closed cycle, also known as “Anderson cycle”. In this system, the hot water from the sea surface transmits heat to a fluid of a low boiling point, and evaporates at determined pressures, near 10 bars. This vapour makes a turbine turn. Moreover, the cold deep sea water is used to condense the vapour fluids, transforming them again into liquids that continue working in the cycle in the same way.

In next posts we will continue writing about OTEC systems and power plants that use it. This energy, which Jules Verne already saw in 1869, is very far from being completely developed.

The following video is an explanation of OTEC. The fluid that they evaporate and use for the cycle is ammonia.

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Ocean thermal power

Today, we are going to provide a general overview of what is ocean thermal energy and its basis. On forthcoming posts, we will continue talking about the technology it is relied on, its importance nowadays and whether it is an advisable way of generating electricity.

So, let’s start from the beginning.  As it is widely known, solar radiation gets in contact with Earth’s surface and this absorbs part of the mentioned radiation; more specifically, about the 66% (but there is an equilibrium between the absorbed and re-emitted energy, so that the Earth does not increase its temperature). It has to be taken into account that more than half of the World’s surface is covered by sea water which means that a relevant part of the solar radiation is absorbed by it. However, the process is not so simple.

The solar radiation penetrates through water, reaching a depth average of 100m. Sometimes, if water turbidity is appropriate (turbidity refers to how clear the water is), the radiation can reach thousands of meters, so the variability of the length of the trajectory is wide. All of these facts make the sea the most powerful energy storage of the entire world.

Making things easier than they actually are, we can talk about a ‘layer-divided’ temperature distribution: there are two main layers separated by an interface; the first one is the superior, quite warm and with an even temperature (20-200m depth) and the second one, from 200m on, which is much cooler. As a reference: at a depth of 1500m, the temperature goes around 4ºC and if we consider extreme cases such as an abyss, we can find temperatures slightly superior to the saltwater freezing point. This temperature gradient is the base of the ocean thermal energy.

The negative point of the idea is that to be worth-doing, the gradient has to be at least of 20ºC, so there are some regions specially favorable thermally speaking: the tropical areas.

About energy conversion: the concept is similar to the one developed on conventional thermal power plants, but here, the warm water of the surface is used as a heat source and the water taken from the deepest areas is which plays the role of ‘coolant’. However, there are some remarkable differences between those installations; one of them is the output: it is really low on ocean thermal energy power plants (it goes around 2-3%). Click for more info about energy conversion.

Here a schematic graphic of the working order:

 

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Membranes for osmotic processes

Following what Juncal and Mikel wrote about, I’m writing today about membranes used to obtain power from the Osmosis process. As both of my colleagues mentioned, the main restriction of the development of this new technology is the membranes that are used in it.
Because all in all, the idea is rather simple: fresh water mixes with sea water, elevating this last one’s pressure producing energy.

This article that I found, “Membrane processes in energy supply for an osmotic power plant”, talks widely about the membranes used in the Osmosis.

First of all, mention that the lifetime of a commercial membrane is of about seven to ten years, keeping in mind that it has to be maintained periodically.

The efficiency of the membranes depends mostly of two factors: the water permeability factor and the salt permeability factor. The key of the fabrication of membranes is to produce a membrane that has a high water permeability factor and a low salt permeability factor.

But it is also important for the inner structure of the membrane not to allow significant salt concentration to build up inside it.

There are mainly two types of membranes under investigation: Cellulose acetate membranes and Thin-film-composite membranes. The first one can achieve a power of 1’3 W/m^2 , whereas the second one has known to achieve around 3’5 W/m^2 . Although, I think it is important to keep in mind that the break-even point of the business is around 5W/m^2 .

Therefore, there is still a long way to go…

As a curious piece of data, did you know that to build a 1 MW plant, 200.000 square meters of membrane are needed?

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Osmotic power by vapour

One of the most problematic causes from preventing a commercial development of osmotic power is membranes which are used in the osmosis process. These membranes have to be replaced periodically, every six months, as Junkal wrote in the last post.

However, there is a way of obtaining electric power by an osmotic process that is highly unknown at the moment, and that does not require any membrane. This way is not directly linked with the sea, but it can show us the great possibilities to improve osmotic power that can be used in the future.

This way uses the different vapour pressures between two liquids. In an experiment done with brine, and always according with this article, when a vacuum connects a dilute and a concentrated solution of brine, each of them placed on separated compartments, the dilute solution evaporates and then it condenses into the concentrated solution. It is possible to take advantage of the transport of vapour that is produced by this way, setting a turbine between the compartments. The system is interesting and there is no membrane, although there are other problems: the process causes temperature changes that have to be solved, and the evaporation tends to equalize the concentrations of the two solutions, stopping the process.

It is difficult to find an image about the process because it is quite unknown. Taking into account that the two compartments are connected by a vacuum, a simple graphic about the system can be the following:


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Blue Energy

Introduction: the osmotic process.

In the osmotic process two solutions are involved and each of them has its own salt concentration (if the salinity is the same for both it has no sense talking about the osmotic process, as it does not happen). Frequently, those solutions are freshwater and salt water; They are put in adjacent containers, separated only by a semipermeable membrane.  This membrane is artificial and works as a filter that only allows the small molecules to pass (such as fresh water). As a result of this solution flow, the pressure in the side of salt water increases and its salinity decreases. This ‘extra pressure’ is the one that will be employed in the generation of electric energy.

 Here, a short video explaining the osmosis process:

There are many processes that use this osmosis effect. For instance, our body uses it to bring water back from our kidneys and plants also use it to maintain a determined water pressure inside them. (Sources: 1, 2)

 

What is blue energy?

It is an osmosis-based alternative energy. There are several ways of generating electric energy from the previously explained process, but on this post we will focus on the one known as ‘pressure retarded osmosis’ or ‘salinity gradient power’. This method was invented by Prof. Sidney Loebin in 1970´s. It uses sea water and fresh water to create that ‘salinity gradient’. The operation is quite simple: seawater is pumped into a pressure chamber where the pressure is lower than the difference between fresh and salt water pressures. Fresh water crosses the membrane and increases the pressure of salt water which makes a turbine go around to compensate the ‘extra pressure’. In other words, the pressure drives the turbine and feeds the generator that produces the electrical energy.

This method has already been operational. What’s more, Statkraft company built the world’s first prototype on the Oslo fiord (Tofte) in November 2009. At first, it started producing only around 4 kilowats, but this sum has increased during the last years and the target is to reach the 25 megawatts in 2015. The enterprise started research in 1997 and went ahead with the patent in 2003. (More info here)

The power plant has required and investment of around 18 million $.

Here a video which graphically explains the work of the machinery and shows some images of the real plant.

Pros and Cons of the blue energy.

It is advisable as…

–          It is renewable

–          There is no risk of running out of salt (the process does not consume the salt, it only uses it to make water move)

–          Minimal environmental impact

However…

–          The costs are really high and that makes the energy expensive.

–          There are many engineering problems: such as the short life of the membranes (6 months), or the slow building of the plants (some methods require building them underground or anchored on the bottom of the sea).

In future post we will develop further those other ways of making the most of the osmotic process

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Santoña’s wave farm

Last post I wrote about a buoy produced by Ocean Power Technologies, a device that produces electrical energy from the vertical movement of the waves. Well, one of the places that they’ve implemented this innovative system is in Santoña, Cantabria.

Santoña is a village of twelve thousand inhabitants that is fifty kilometers away from Santader, the capital of Cantabria. This is where Iberdrola Renovables decided to make the investment  to implement OPT’s Power buoy.

The project started in July 2006, when OPT formed a joint venture with Iberdrola and other institutions. But it wasn’t until October 2008 that the first buoy was installed.

They installed the PB40, a buoy that produces 40kW. The plan is to install another 9 buoys, but PB150s, that produce up to 150 kW.  Therefore, the total  energy that the plant would produce would be nearly 1,5 MW. Once the whole wave farm is completed, it will produce enough energy to supply 2.500 households.

This farm is 4 km off the shore, so it has no visual impact what so ever. But being so far away is a problem for the engineers: they have to install cables to be able to bring the energy to the electricity on to the mainland. This is still to be done, so this project is long from still being finished…

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