Fluidyne Pumps

A good question to ask when reviewing a description of efficiency is 'what input is the basis'?

The follow up question is how available/what is the cost of the input.

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Since you are contemplating something that involves some land use for pumped storage, it is unlikely that the sun exposed square footage is likely to be your limiting factor. As such, efficiencies that you related in rough percentages are bound to be misleading.

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Unless the total usable sun exposed area has already been accounted for in various uses, the efficiency per KW from sunlight is only of indirect consequence.

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The efficiencies that you allude to when you mention things like few moving parts and low maintenance along with efficiencies related to capital costs and expected life are probably going to be far more helpful.

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Estimates of lifetime useful output per lifetime cost is a better consideration of efficiency.

As fluidyne pumps are so inefficient you may wish to consider a more efficient alternative: a solar brine pond in conjunction with an organic Rankine cycle (ORC) engine. The pond acts as both the solar collector and the energy storage medium to tide you over on cloudy days. The energy obtained is in the form of low-grade heat of 70 to 80 °C compared to an assumed 20 °C ambient temperature. According to the second law of thermodynamics (see Carnot-cycle), the maximum theoretical efficiency of a cycle that uses heat from a high-temperature reservoir at 80 °C and assuming an ambient air temperature of 20 °C is 1−(273+20)/(273+80)=17%.

With the proper choice of working fluids an ORC can be also used with your parabolic-trough collectors directly.

Welcome to CR4, btw.

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It's quite clear that I've come to the right forum. I remember reading about brine ponds a long time ago but had forgotten about that. I didn't realize that the water would get as hot as 80C. Even with my limited amount of research so far, I'm betting 0.17 efficiency is simply unattainable with a fluidyne pump.

My next question was going to be concerning liquids with a lower boiling point than water. I do know that the volume expansion when the phase change from liquid to gas happens is pretty extreme. Will study up more on the Rankine cycle.

Thanks for the nudges in the right direction. Thanks also for your collective patience with a curious layman.

The inefficiencies of your proposed system will make it so ineffective you will be wasting a lot of electricity in trying to get a small amount. For each step that you change from one energy generating step you are losing energy to the equipment to run it. By pumping the water from a lower elevation to a higher one requires the theoretical pump power needs and also the inefficiency power losses of the pump. (Elevated stored water is intended only to shave or store for peak power usage generation). The turbine also has inefficiency losses. The piping has friction losses. You are going to have an extremely inefficient system.

Why not just take the initial solar cells, which are part of the system, and use that electricity? The subsequent losses after that step would be eliminated. There is no or very little loss in using that as the source. Everything else robs the system when final available power considered. A quick estimate is that the efficiency of the system as you have described it will be less than 5% or more likely less than that.

Good Luck, Old Salt

Old Salt,

I missed the parts about solar cell or other inputs of electricity.... where is that?

Thanks Old Salt, I think your answer is a good one as well.

I agree with your analysis. If electricity without burning fuel is the end goal, it will be hard to compete with mass produced solar cells and/or wind turbines on a lifetime KW-hr/$ basis.

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I think there might be some niche uses that a fluidyne pump might serve well though. One of those niche uses would be reliable, low maintenance, minimal initial and total cost, water pumping.

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Also, while a focused optics paraboloid dish would need a tracking system continually making adjustments, there are options for non-focusing optic semi-parabolic troughs aligned east-west that might require slight seasonal adjustments or possibly none at all. Since fluidyne pumps work well with a concentrated heat source, it would make sense to have a number of these along the trough in series-parallel arrangement as needed.

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It wouldn't provide any advantage for generating electricity, but there is potential for a robust inexpensive system that could pump for years with very little upkeep.