Low Temp Solar Piston Brayton Cycle with Pebble Thermal Storage

Found this guys work on low to medium temp solar cycles to be compelling. http://sunoba.com.au/BRRIMS%20flowsheet.htm
Curious if any on this forum, such as Marcos Buenijo might care to weigh in on the possibility of implementing this design on small scale home built system.

I'd skip the reheat as the complexity is a bit much for the small scale but would recuperate exhaust heat as well as capture excess daytime heat for nightime operation. I think that the insulated updraft pebble bed energy storage scheme looks like an excellent way to store energy without requiring a heat exchanger to recapture.

The engine design is the large challenge here, but the pressures would be extremely low. My hunch is low speed design of <200 rpm would be the way to deal most efficiently with such low pressure gradients Valving would be tricky, may need to be able to control timing on the fly so as to optimize pressure ratios for varying temps. If collection scheme was largely passive engine might be made high temp plastics or acrylics.

Lots of engineering work no doubt, but its an interesting cycle that could overcome many of the limitations of a stirling and with similar efficiency.

Hi Nick. I missed this one. I agree that a modest pressure and very low engine speed would be required to make this viable. Reheat is a complexity, but it sure does increase efficiency a great deal.

When I first started looking into small scale heat engines for residential scale cogeneration, I considered solar heat first. However, since that time it's become clear to me that fighting photovoltaics for small scale power generation in the off grid setting is a losing battle. The basic idea considered at the link provided is very similar to many ideas I have considered. I like the idea, and it's clear that the idea was considered by someone who understands heat engines well. I like the thinking that went into it, but what's required is a cost effective way to make it happen. There are no details along these lines provided. I'm skeptical without this information.

The main problem I see is that even modest temperatures will see high thermal losses. I would expect anything approaching 200F to see thermal losses on the order of 50%. The only way I see to reduce these losses would add substantial cost. This is one of the main reasons I started looking into biomass for off grid (i.e. remote) cogeneration applications.

Marcos,
I agree PV is most cost effective at present. However i think it'd be unfortunate if CHP solar thermal options such as this are not explored as distributed solar alternatives. Pv tech will remain wedded to the industrial complex for the for-seeable future, and is both less resilient and autonomous than a simple heat engine could end up being.
IMO what is needed are several open source crowd funded experiments to flush out the feasibility of attractive options like this one. These engines are operating in a completely different manner than there energy dense IC cousins and need to be designed to cost effectively around the low operating temps and pressures. Piston and cylinder design may not be optimal at all, perhaps a "bellows" style piston would prove more cost effective given the low pressures and high volumes.
As far as efficiency goes you are of course right electrical efficiencies will be low at those temperatures, but you do have a system producing heat which minimizes those losses. Also without getting into the specifics of moedlling what the efficiencies will be I'd like to emphasize that efficiency is not the main metric to consider in designing these systems, overall cost per kwhr is. A simple low tech system operating at 5% efficiency may end up being superior in terms of providing people with a resilient and autonomous energy system to the more efficienct and complex PV or biomass system.

Nick Raaum wrote:Also without getting into the specifics of moedlling what the efficiencies will be I'd like to emphasize that efficiency is not the main metric to consider in designing these systems, overall cost per kwhr is. A simple low tech system operating at 5% efficiency may end up being superior in terms of providing people with a resilient and autonomous energy system to the more efficienct and complex PV or biomass system.

I agree with this. Overall efficiency is not as important to the majority of the world that can't afford PV tech right now.

The other issue here is that there is considerably more low-temp heat available to us than solar energy. A lot of the accessible low-temp heat sources are waste products from other systems (like IC engines), and even if you are only able to capture 5-10% of that energy, it amounts to a significant increase without extra fuel costs.

I would love to build a low-temp system like this, but there is a real lack of detail and information for the backyard builder, like myself.

Nick Raaum wrote:Marcos,
I agree PV is most cost effective at present. However i think it'd be unfortunate if CHP solar thermal options such as this are not explored as distributed solar alternatives. Pv tech will remain wedded to the industrial complex for the for-seeable future, and is both less resilient and autonomous than a simple heat engine could end up being.
IMO what is needed are several open source crowd funded experiments to flush out the feasibility of attractive options like this one. These engines are operating in a completely different manner than there energy dense IC cousins and need to be designed to cost effectively around the low operating temps and pressures. Piston and cylinder design may not be optimal at all, perhaps a "bellows" style piston would prove more cost effective given the low pressures and high volumes.
As far as efficiency goes you are of course right electrical efficiencies will be low at those temperatures, but you do have a system producing heat which minimizes those losses. Also without getting into the specifics of moedlling what the efficiencies will be I'd like to emphasize that efficiency is not the main metric to consider in designing these systems, overall cost per kwhr is. A simple low tech system operating at 5% efficiency may end up being superior in terms of providing people with a resilient and autonomous energy system to the more efficienct and complex PV or biomass system.

Hi Nick. I suppose I agree with you. In fact, I've made similar arguments in other contexts. There are a few companies working on micro organic rankine cycle engines that use piston expanders. There is one using a scroll expander as well. Personally, I prefer the piston expander. It's simple, and it can be made to see extremely high expansion in the right configuration. BTW, I do believe the organic rankine cycle is the best way to go about a solar heat engine. One company (in Sweden I believe) seems to be putting some serious money behind a unit: http://www.vdg.no/index.php?menuid=16. I only recently became aware of their work, so I don't have specifics on their unit. It is a piston expander. I caught glimpses of schematics in a video they released, and it seems to be a two cylinder single-acting expander without compounding, cam operated poppet valves, and possibly a cylinder jacket for heating. That's probably the best way to get high efficiency at low cost with temperatures limited to about 400F. People could really use a rugged heat engine like this if mass produced and readily available at a reasonable price. Based on my research, I believe the overall electrical efficiency of this kind of unit limited to 400F working fluid temps could approach 10% (not including thermal losses from the collector), and should achieve 5% when these losses are factored. Direct mechanical energy could be used to drive an efficient heat pump for cooling, and this would avoid a lot of the energy conversion losses that cannot be avoided in electric motor driven vapor compression cycles. Add to this the heat delivered from the cycle for heating applications, and yeah, I think it has some potential.

My personal preference remains making full use of biomass at the residential/personal scale. The most promising technology for this would be a modern piston steam engine (my opinion) with emphasis on high thermal efficiency and efficient heat recovery. However, until we can see this, then I think a wood gas engine system is the best thing going. If someone wants to unplug today, then a combination of photovoltaics and a wood gasifier for heat and power, and with a lead acid battery and thermal mass storage (such as heated water), seems the most practical alternative. Steam has the potential to be superior, but without modern and mass produced hardware that prospect has to remain an idealized consideration.

My personal preference remains making full use of biomass at the residential/personal scale. The most promising technology for this would be a modern piston steam engine (my opinion) with emphasis on high thermal efficiency and efficient heat recovery. However, until we can see this, then I think a wood gas engine system is the best thing going. If someone wants to unplug today, then a combination of photovoltaics and a wood gasifier for heat and power, and with a lead acid battery and thermal mass storage (such as heated water), seems the most practical alternative. Steam has the potential to be superior, but without modern and mass produced hardware that prospect has to remain an idealized consideration.


The only problem with biomass is that overall cycle efficiencies are bottlenecked by the photosynthetic efficiency of the plants, which is probably around .1-.2% efficient. So its a fine solution for using woody wastes or sensible energy biomass crops (populars ect), but it'll never really scale anywhere near the orders we'd need in the long run. I tend to think in the long run biomass energy is for transport only and one way or another we'll have to use solar for electrical production. That is likely much too long range thinking though to be practical and if you are just looking to unplug now I'd agree with your approach. Still the optimist and idealist in me wants to figure out whether a low tech solar solution is feasible now.

I do agree the organic rankine cycle has potential and would yield higher efficiencies than an a brayton cycle at the same temps. Yet I want something simpler and easier to operate and replicate.

Nick Raaum wrote:The only problem with biomass is that overall cycle efficiencies are bottlenecked by the photosynthetic efficiency of the plants, which is probably around .1-.2% efficient. So its a fine solution for using woody wastes or sensible energy biomass crops (populars ect), but it'll never really scale anywhere near the orders we'd need in the long run. I tend to think in the long run biomass energy is for transport only and one way or another we'll have to use solar for electrical production. That is likely much too long range thinking though to be practical and if you are just looking to unplug now I'd agree with your approach. Still the optimist and idealist in me wants to figure out whether a low tech solar solution is feasible now.

I do agree the organic rankine cycle has potential and would yield higher efficiencies than an a brayton cycle at the same temps. Yet I want something simpler and easier to operate and replicate.

True and true. Net efficiency of biomass is poor, and I'm focusing only on individual energy independence. Direct solar is the best solution with respect to long term.