Nexus peripherals?

renewable-energy

#1

I understand this is a bit premature in its thinking…but I wondered if other users on here had application work in mind such that some of the promise of the nexus had some practical demonstrability/realization. The ability to run a heat engine is kind of interesting but, so far as I can see, the product that can either run on low temperature steam or at high enough wattage to handle most workloads doesn’t seem to be there. most steam engines have trouble with anything below 800 degrees and most Stirling engines don’t get bigger than a few dozen watts.

I get that these are made for more than one use case in mind…for my part, I was thinking a nice water heater…but I just wondered if anyone else had the practical demo for electrical production worked out. perhaps a dewar flask that could be added into the already in place vacuum circuit for hot resevoir holdover storage or similar?


#2

The the theoretical maximum efficiency of any heat engine (steam, stirling, gasoline, diesel or gas turbine) can be calculated using the simple equation for Carnot Cycle efficiency, E= 1-Tc/Th where E is efficiency, Tc is the cold side temperature and Th is the hot side temperature (temperatures must be absolute, ie Kelvin or Rankine). From the equation it is obvious that hotter temperatures on the hot side Th will result in higher efficiency, so you are correct that 800 degree Celcius (1073 Kelvin) steam will give higher efficiency than 200 Celsius steam (a reasonable temperature for the Tenkiv nexus while still achieving good solar collection efficiency). But it is instructive to run the numbers to see what actual efficiency we are talking about.

We will assume a cold side temperature of 27 Celsius (300 Kelvin) for our heat engine.

At 150 Celsius above ambient Th (450 Kelvin) the max efficiency will be 1- 300/450 or 33%. So a perfect heat engine running on a Tenkiv Nexus collector temp of 150 Celsius above ambient (the Tenkiv Nexus will achieve about 65-70% efficiency in full sun at 150 Celsius above ambient) could achieve an overall efficiency of 33% x 70% or 23%. This is a bit better than very good commercial PV panels.

A heat engine running at 1000 Celsius (1300 Kelvin) Th could achieve a max efficiency of 1 - 300/1300 or 77%. So an ideal steam engine running at 1000 Celsius and with a condenser temperature of 25 Celsius could achieve 77% efficiency.

The reason we don’t see steam power plants running 77% efficiency (actual coal or natural gas fired steam power plants run more like 40% efficiency) is because the Tc is not 25 Celsius (its much higher), much of the energy released in the burning of the coal of natural gas goes out the exhaust stack (the combustion temperature of natural gas is about 1950 C so if Th is 1000C roughly half of the heat will be wasted (unless one uses a combined cycle plant, where some of this waste heat is recovered). A solar powered heat engine does not have combustion products that must be dumped so the losses can be much lower.

The bottom line is this: 1. A well designed heat engine running off of the Tenkiv Nexus could see 20-30% efficiency.
2. You need a heat engine designed to run at a Th that can be achieved by the Tenkiv Nexus at good efficiency (roughly 150-200C). There are ORC (Organic Rankine Cycle) and Stirling engines that are designed for these temperatures in sizes from a few milliwatts to many 10s or 100s of kilowatts.
3. You could also use a TEG (thermo Electric Generator) but these typically have very low efficiency (a few percent).


#3

Oh, I didn’t mean to imply that the overall system efficiency wasn’t great. I guess, if anything that is one of the nice things about having (essentially) free process heat available; it can be inefficient thermodynamically, but the input is nearly free so a device of even modest efficiency can run without much concern for the system efficiency. such is the benefit with absorption refrigeration etc…which again, this could power. I was merely looking at it from the electrical side

My issue is that a suitably scaled steam engine or (more likely) Stirling engine doesn’t appear to be out there to handle an input such as this where the steady state input energy could easily be 5-6kw on a 3-4 unit setup running parallel loops. Though I suppose certain things like old lister engines might be able to be repurposed for the cause. TECs/TEGs remain an interesting proof of concept but the per watt cost and performance put it out of the practical realm for now.


#4

Below is a list of links to some small heat engines that are presently available. Tenkiv is also developing our own ORC engine specifically designed to work at the temperature differentials at which the Tenkiv Nexus would be most efficient.

http://myriadpower.com/rankine.html
http://www.infinityturbine.com/
https://www.researchgate.net/post/Does_someone_know_companies_selling_small_ORC_10kWe

The problem with all the solutions that are presently available is that they are semi custom or very small scale production items, so cost is high and efficiency is likely to be less than ideal.