Physical Insights

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ThermoGen: When “Green” energy doesn’t add up.

with 3 comments

I’ve been looking at some of the claims on the website of Thermogen recently.

In short, what Thermogen claim is that they can supply a domestic solar thermal energy installation whch provides an electrical power output of 5 kWe, and that that power output is accessible 24 hours a day via energy storage in tanks of high-temperature water.

The Thermogen is designed to supply it’s rated output 24 hours per day, cloudy or sunny weather e.g. a 5kW system will supply 120kW per day. [sic] It has a three day design storage for inclement weather.

Of course, if a 5 kW system can supply 5 kW of power for 24 hours, then that’s 120 kilowatt-hours of energy. I’m a little bit wary of a company selling energy technology when they can’t tell the difference between a unit of energy and a unit of power.

This heated water is then stored in 1000 litre insulated tanks at 150-200 °C. These tanks are a solar energy storage system designed to store enough energy to provide the following services for up to three days without sunshine:

For a 5 kWth system to be able to store energy up for three days, then 360 kWhth of energy must be stored in the system. We’ll come back to that in a minute.

They explain that:

Each panel measures 2.4m wide and 2m up the roof. It is expected that you will need 7 of these panels for the Thermogen system.

That’s a collector area of 33.6 m2. A little less than that, actually, since not 100% of the panel’s dimensions will be usable solar collector area.

For comparison, a standard large two-panel Solahart hot-water system has a collector area of 3.5 square metres.

Now, if we look at BOM’s map of average daily solar exposure across Australia, we see that the average daily solar exposure is, in the sunniest parts of nothern Australia, 21 megajoules of solar energy per square meter per day.

If there is one idea to keep in mind when considering solar energy, that is it. Irrespective of what sort of collector technology you have, there is always a very finite limit to the amount of energy you can collect from a solar collector of a given area. That energy flux is the maximum that there is to be utilised, no matter what technology you use to harvest it.

So, anyhow, we have 21 MJ/m2/day, and a rooftop solar collector of 33.6 m2. We’ll be conservative here and assume (a) you live in Townsville or Alice Springs or Darwin and (b) the entirety of the surface area of those collectors is active area. So, the power output that you get is a maximum of 705.6 megajoules per day.

The most efficient evacuated-tube solar thermal energy collectors, like the ones proposed by Thermogen, manage a gross efficiency of energy collection of about 60%. So, now we’re down to 423.36 MJ per day.

This thermal energy is then converted into electrical energy in a heat engine. In this case, the engine that they’ve pictured on their webpage – without attributing it’s source – is a Freepower 6 [PDF link] 6 MWe Organic Rankine Cycle powerplant.

Some of the images on the Thermogen site appear to depict the FreePower 6 organic Rankine cycle engine / generator as well as a Rotartica absorption chiller, with no credit given to the peope responsible for these components.

(An organic Rankine cycle is simply a Rankine cycle engine using an organic chemical as the engine’s working fluid, such as a fluorocarbon liquid, a fluorocarbon gas like a refrigerant type material, or some type of liquid with a lower boiling point than water, as the engine’s working fluid. Such engines are commonly used to recover low-grade heat from industrial processes, and as geothermal electricity generators, since they’re designed to operate with low temperatures.)

Now, let’s look at the specs of the FreePower 6 engine. It requires a heat source of 180 °C, returns the cooled oil at 123 °C, and requires a thermal power input of 70 kWth, to generate an electrical power output of 6 kWe. Since the Thermogen system is supposed to generate 5 kWe, I presume 1 kWe is consumed to drive the hot water pumps.

Therefore, the engine only has an efficiency of 8.6% – seemingly very low efficiency indeed. That seems like a terribly low efficiency, but the maximum efficiency – as per Carnot’s theorem – at these temperatures is only 11.6%, so the efficiency realised in practice isn’t too bad. At least these guys aren’t trying to flog off a perpetual motion machine.

Now, if we’re putting 423.36 MJ into the engine, and our electricity output is uniform day and night, then at this efficiency, we have a thermal input power of 4.9 kW, and we’re getting an electrical power output of 421.4 Watts.

I suppose that might be where they’re getting their claimed of “5 kW” from, given that they’re putting an average power of about 5 kW thermal into the engine?

Furthermore, cooling water is required to dissipate the engine’s 60 kWth of waste heat, at a flow rate of up to about 0.8 L/s and a maximum outlet temperature of 55 °C. I’m afraid that yes, in this house, we obey the laws of thermodynamics. Perhaps you need to build an artificial pond next to it or something? But that’s good, right? Lake Anna attracts lots of tourists, doesn’t it?😉

So, we’re left with 421.4 Watts. Jump for joy; your energy needs are solved forever.
That’s, what, enough energy to run a handful of incandescent light bulbs?

We have an electricity output of 10.11 kWh per day, then.

The energy requirement for an average home is 10kW per day (See Synergy website), so the additional 110kWhrs of electricity may be supplied to the grid.

For example, a 5kW Thermogen system will generate 120kW per day while the average Australian home uses between 10kW to 30kW per day.

Average Australian household electricity consumption is about 15 GJ per household per annum – 11.4 kWh per day.

If you have a small household, or an energy-efficient household, then such an installation can realistically meet all your household electricity needs. Probably. If you live in northern Australia. If you live in Melbourne, Sydney or Adelaide, forget about it. Even if you could supply all household demand for electricity, however, there will be little or none left to sell into the grid.

They claim that the power generated from a domestic Thermogen installation “will supply a revenue stream of up to $20,000 per annum at current rates which will pay the mortgage on most homes in Australia”.

It won’t.

If we really did want to generate 120 kWh of electricity per day, what would be required? You’d simply need 400 square meters of collector panels. That won’t fit on your roof. It would be the kind of system that lives up to what they’re claiming, though.

This is before we even start thinking about the energy storage tanks of a couple of thousand litres of water at 150 to 250 °C – superheated water at pressures exceeding 200 psi. If you’re standing around the tank and it ruptures, it will cook you to death. Do you really want this engineered and installed in homes by people who can’t tell the difference between power and energy?

For a 5 kW system to be able to store energy up for three days, then 360 kWh of energy must be stored in the system.

If the initial temperature of the water is 180 °C, and the final temperature of the water is 123 °C, then the storage of 360 kWhth to supply three days worth of energy requires 360 kWh / (4.184 Jg-1K-1 * 57 K * 1 g cm-3) = 5434 L; 6 quite large 1000 L storage tanks.

How much does all this cost, anyway? There is not one word of it on the site thus far.

There’s nothing especially malicious or ill-intentioned about Thermogen – although I would not invest in them under any circumstances. They simply appear to be another trendy, hopeful “Green” enterprise that simply can’t count.

The illustrious Dan Rutter has more in a similar vein, here.

Written by Luke Weston

October 11, 2008 at 11:55 am

3 Responses

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  1. Yeah … poor math. Sadly, it’s not uncommon.

    Luke, have you ever considered giving a critique of the “vehicle-to-grid” (V2G) concept?

    I can see how having electric vehicles drawing power from the grid during down times (say, at night) could have a beneficial, stabilizing effect — essentially leveling out the load — particularly if a smart system could be designed so that the car charges when demand is low and electricity is relatively cheap. However, that is “grid-to-vehicle” technology. I have yet to hear a reasonable argument for the energy flowing the other way.

    In other words, energy that is available in a portable form — say, in an electric car’s expensive batteries or as petroleum fuel — always includes a premium for that portability, which is why the world avoids burning oil for electricity these days, and where it is burned, the electricity is very expensive. So if energy is available and ready to power a car, why would anyone want to dump that expensive energy to power the grid, where any power source will do and there are plenty of cheaper, less portable sources?

    Even if we want to subsidize suburban homeowners to “feed the grid” and adjust the electricity market to reward customers for providing electricity into the system during times of peak demand, how does doing this with one’s car (which might not be available when needed, and which would need to use its expensive batteries to do so, thereby reducing its range from its electric motor in the near future) make more sense than, say, installing an industrial-sized battery (which is cheaper per kW) in one’s garage? But if that is such a good idea, then why wouldn’t utilities build huge battery banks to do the same job much more cheaply?

    Anyhow, your thoughts would be appreciated.

    bryfry

    October 11, 2008 at 5:37 pm

  2. I think with cold water washing and ‘navy showers’ a household can get by with perhaps as little as 20L per person of 45C water in storage. If thin film solar photovoltaics live up to their promise of costs down to say $3 per watt the question becomes how best to use rooftop energy capture. Options include battery storage, hot water storage, ice storage, selling back to the grid, charging an electric car, running a ground sourced heat pump and so on. The optimum choice would seem to depend not only on spending limits but ongoing developments such as the price of grid electricity and fuels for heating, cooking and transport.

    Carbon pricing and carbon revenue redistribution is probably necessary to accelerate this learning process. Unfortunately governments who have promised to take this path seem to be losing their nerve.

    John Newlands

    October 11, 2008 at 10:36 pm

  3. The error you’re making is that you assume that any heat not used by the stirling engine immediately is gone.

    It isn’t. It’s still in storage. In fact, it remains in storage until it is either used or leaks out.

    I accept that this system may have some issues with overheating in the storage unit – it’s going to get much hotter than a mere 200 degrees.

    But, lets face it, whatever energy goes into storage is coming out one way or another – either as electricity, as leaked heat or as a massive high-pressure dry steam explosion …

    … most likely the later. There are better alternatives for storing heat, but I’ll keep those to myself😉

    Ian

    December 16, 2008 at 7:24 am


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