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  6. Double-Pipe Heater Exchanger

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- [Voicover] So more recent developments are from conceptual models that have been done about methodology that is called the double heat pipe exchanger as you can see in this diagram, the idea is to use very deep wells that have relatively high bottom temperatures, it's especially suitable for abandoned oil wells where we can install external pipe as you can see here that external pipe will pick up the heat from the formation, from the rock, and then an internal pipe that can take the fluid out of the system, in other words we are going to have injection of the fluid in the outer cylinder the water is going to circulate downwards and pick up the heat from the formation and increasing the temperature and then finally the bottom of the well is going to be pumped out through the second pipe, the internal pipe. Okay this is a good idea and first it was proposed to use water as the circulating fluid but later on, in a recent paper Davis and Michaelides have proposed that instead of using water we can use a secondary fluid for example butane, iso-butane excuse me or any other fluid that can work with the Rankine cycle. So here we have again, the same structure of the pipe, we can inject the fluid here, the fluid is going to boil, in other words the heat is introduced into the system and then we are going to extract here the hot gas that can later be used to generate electrical energy. This is a very, very promising idea because we aren't going to need in this case the pump and the heat exchanger that we use in the Rankine cycle in other words, the well itself is going to be the heat exchanger in the Rankine cycle so here you have some of the information, the power produced by the double heat pipe exchanger depends also on some parameters, for example it needs to model for the even down-hole temperature of the heat well, the output energy should depend also on the injection pressure and injection velocity, it depends also strongly on the geometric characteristics of the pipe, depth of the well, inner and outer pipe radius, thickness of the inner pipe, in other words the insulation layer between the two pipes, the properties of the fluid are also of similar importance because we know already that the net power output depends on the type of fluid, pressure and temperature, in addition, this has not been modeled yet but it should depend on the thermal properties of the surrounding rock and water. In the case of the two models for water and for iso-butane that have been done it is assumed that ground water velocities are practically zero, in other words we are in the presence of a completely conductive system. But in real life we can have situations where that's not going to be the case but again it's not extremely logical to do modeling and get results to see what is going to be the final energy you can extract from this well. Some of the advantages of the double-pipe heat exchanger are for example in the study that they did with the iso-butane the modeled resource indicated that you could produce between two and three megawatts if the wells depth is around 3 thousand meters and the bottom-hole temperature is 450 degrees Fahrenheit. These are typical conditions that you find in the south Texas wells. So there is possibility there to obtain a lot of energy. One advantage also is that the power is not intermittent as you have with other renewable energy resources like for example solar energy, the need for an external pipe and heat exchanger is eliminated, as I mentioned before because the well itself is going to be the heat exchanger in the Rankine cycle, the maximum flux, it minimizes the flux of greenhouse gases basically we're issuing half the emissions of greenhouse gases, except maybe for some accidental release of the secondary fluid but in general it's a very clean energy type. And then the most important thing is there's a great potential for the many abandoned wells in the United States for this kind of technology, there are some concerns also that we need to point out we need large volumes of the secondary fluid obviously depending on the dimensions of the well, we have to consider that we have to fill up practically the well with the secondary fluid and then we have to use it also in the external pipe to transfer the well fluid turbines and so on, the price of these gases can be high in that case we have to find what is the optimum gas to be used depending on the pressure temperature also obviously the price of the gas. Another point that is important here is that we need to eliminate any leaks between the two pipes, we don't want mixing of the fluids from the external to the internal pipe. And then not only leak of the fluids but also heat so the thickness of the internal pipe is important because it should guarantee that we don't have, and also the material of the internal pipe, so we need to guarantee that there is not going to be extreme transfer of heat between the internal and the external pipe.