Towards a better tinyhouse

Inventing to freedom?

Talked to Sunpower today about those engines.

with 4 comments

I talked to Sunpower today. Very interesting, unfortunately I was sort of caught off guard by the phone call and was sort of incoherent. But I sort of found out what I was looking for. Unfortunately the 90 watt ones are made for use in space (radioisotope generators?) and are several thousand bucks a pop (btw the thousand watt one they have is about $30k.) Apparently they last for 20 years with no maintenance (I assume due to the air bearings and IIRC the planar spring took some figuring out).

But the guy I spoke to said the company was interested in a partnership with a company to produce a lower cost product. My impression was that they know how to produce them at much lower cost but they assume there are no customers right now. I don’t know what kind of price, I would think they could get it down to the $3-$5 per watt range but I didn’t remember to ask him. I have no idea what sort of volume they would need and I didn’t want to waste his time because I am of course not in any postion to be a customer for more than a few of them right now ( I was hoping there were reasonably priced ones in production already) (BTW don’t call them if you are an individual like, me I’m sure they won’t appreciate the waste of time, but the pricing information was not on the web anywhere so I had to call them. Now I’m posting it on the web though so you know, and don’t have to call (actually I would prefer if he gave me a dollar figure, but he didn’t seem to want to, maybe they make each one custom)).

But he seemed surprised when I said an 90 watt engine would be worth maybe $1000 to me. It’s true though. Why? Well I previously mentioned the idea of a heat engine plus a solar panel being a particularly interesting combo. At the time I assumed the engine would only run at night to reduce wear and tear, but now if they can last for many years without maintenance, I’m thinking what would happen if you had a generator only, and replaced the solar panels with it?

I figure a finished generator could be worth roughly $40-$30 per watt in terms of breaking even with solar panels in a conventional solar power system. Of course it would be better to save some money though, but there is the environmental and practical benefit of reducing the size of the batteries too, and it’s pretty much the only way to do so aside from saving energy, which only gets you so far. I figure as follows:

Suppose you look at a typical off grid solar power system ( and in most areas that is cheaper than wind). One that comes to mind which I happen to know has the pricing listed for it is this one:

Kyocera KD210GX-LPU 210 watt solar panel:
$590 each x 10 = $5,900

(those 195 watt solar panels I was using as an example in the cheaper photovoltaic thing: 2100 watts at $1.69 per watt= $3,549 if you were stationary and could use other panels instead)

Concorde PVX-2120L 212Ah AGM battery:
$490 each x 8 = $3,920
Outback Flexmax 80 charge controller:
$599
Outback VFX3524M 3500 watt inverter/charger:
$2,029.51
Outback MATE systems display and controller:
$239
Outback Power HUB4 communications manager:
$155.61
Outback Power Flexnet-DC system monitor:
$322.15
Outback Power FWPV-8 array combiner:
$118.15
Samlex DC to DC converter:
$74.50
Various other standard panels, wiring, outlets, etc.

Total cost for listed items:
$13,357.92

5 Kwh per day is an average of 208 watts.

So what would the cost of the system look like with a 208 watt generator? Well the solar panels would be gone. You still need some battery, but not nearly as much because you don’t need to accommodate cloudy periods or the night as much. The amount of energy from the battery you need over time period x is the amount of energy the user uses minus the amount the generator produced. Suppose you draw power at 2 KW for 2.5 hours worst case scenario, using all the day’s power up in a short time like that, that’s 5KWh-0.208KWh*2.5= 4.48 KWh. The batteries are 12volts for 2544 Wh each but you only really can use 70% of that so 1780. So you’d only need 2.5 batteries (supposing you can get a half size one at about the same price per Wh) although personally I would probably use a lot less because that worst case is not very likely and even if you did run out it’s no big deal because you will always have at least as much power as is coming from the generator for lights etc I might even use one or half, just enough so it can support the normalpeak loads, but then that is me, and we should stick to what people who are actually doing it would apparently use, given what they seem to be doing.

Now the system pricing looks like:
Concorde PVX-2120L 212Ah AGM battery:
$490 each x 2.5 = $1225
Outback Flexmax 80 charge controller:
$599
Outback VFX3524M 3500 watt inverter/charger:
$2,029.51
Outback MATE systems display and controller:
$239
Outback Power HUB4 communications manager:
$155.61
Outback Power Flexnet-DC system monitor:
$322.15
Outback Power FWPV-8 array combiner:
$118.15
Samlex DC to DC converter:
$74.50
Various other standard panels, wiring, outlets, etc.
Plus cost of generator
Total cost for listed items:
$4762.92 plus cost of generator

The generator is worth about $8595 or $6244 if stationary or $41.32 per watt or $30 per watt. A 90 watt generator including the burner etc would be worth $3718 or $2701. But you have to pay for the fuel, right?

But the thing is the batteries have a running cost associated with them too. From the references below it looks like they loose at least 8% capacity per year.

Secondly, if you are doing space heating or hot water heating with propane, you can use waste heat of the engine to to those things instead of burning propane (cogen).

Assuming you get 90% of the heat of combustion out of the propane, you get 12587.5 Wh per Kg, assume 28% of that gets converted to electricity that gets used, that’s 3524.5 Wh per Kg of propane so that’s 1.41 kg of propane per day or 518 per year.

We are using the waste heat though, so subtract the amount of heating propane that it displaces. When there is more heat coming from the generator than is needed to heat the house you would then count the additional propane (in addition to what would be needed for space heating the house) as what is used as fuel by this scheme which would not otherwise be used (i.e. if you did it all with solar panels)(and yes using solar heating would be nice, but for whatever reason we can see that is not what people usually actually choose).

So the amount of heating fuel the waste heat from the generator would displace would be about the lesser of the amount of fuel that the generator uses and the amount needed for heating that day (well it would be more accurate to look at each hour rather than each day but I don’t have time to right now).

Assume we capture 90% of the total heat of combustion (which includes the energy from the electricity being converted to heat after being used).

I would need to pull out the spreadsheet to know exactly how much fuel you’d save because it needs to be estimated on an hour by house basis, but looking at that solar heater spreadsheet, the amount of heat a house this size needs on a cold winter day when it’s -10 out (which is typical in January in mineapolis) with a 0.3 air exchange rate with 50% heat recovery (so a cheap aluminum exchanger) is about 16Kwh, and the amount of heat we got was 1.41*12587.5= 17747.67 Wh per day. So it displaces almost all the propane you would have needed in a typical year. In a colder year it would actually displace more Kgs of propane, although your net use those years would be higher. Warmer years the converse.

I’ll just add up the amounts from the spreadsheet… looks like you’d need a total of 2560379 Wh of heat for the year, so 203 Kg of propane, meaning you only really use 518-203=314 per year in a typical year that you would not otherwise have used. Assuming propane is $1.2 per Kg, $377. Not bad at all. Plus if you were using waste heat instead of propane to heat hot water in summer, 40 liters of hot water takes about 1.5 KWh per day average in summer, so you would save a bit of dough there too.

I would argue that heating hot water or space heating with solar is even better still, but most people are in fact using propane for both those things unfortunately.

The ongoing cost of the batteries that are eliminated from the system by the generator for the solar system would be at least 2695*0.08=$215.6.

So the running cost of the system with the generator is really not bad. Factor in the financing dimension and it’s even better. Of course it would be nice if you could use an unusual lower carbon fuel like veggie oil or biomass but that would be another project in itsself to invent a good burner.

See below for global warming impact.

Anyway obviously this sort of thing has a lot of potential if you could get the engines at a reasonable price.

Another thing is that given that fuel costs are not much of a problem, what if we could do this with an engine other than these stirling engines? The problem is maintenance mostly, and the fuel efficiency is a bit lower too. Still, tune in some time in the future, I am thinking about how you might be able to take a cheap generator and modify it to reduce maintenance reqs……

total lifecycle carbon emissions for fuels http://www.biomassenergycentre.org.uk/portal/page?_pageid=75,163182&_dad=portal&_schema=PORTAL
lpg is 90 per GJ of combustion energy, which is provided by 49.1 KG of propane so 90/49.1=1.83 kg of co2 per kg of propane (not bad actually).

In terms of global warming, this site says it takes “75 kilograms of CO2. In the worst case scenario, that becomes 314 kilograms of CO2″ to produce a solar panel”. That’s a pretty vague estimate, and surely it varies depending on the type of panel (we are assuming it is a silicon panel though since are the only ones we can buy). But still they are not free in this regard. There were 10 panels and they are 1.49 sq meters each so about 14.9*75=1117.5 kg of CO2, equivalent to 610 kg propane. So I guess it could be worse after the first several years if you use propane, but you could use a carbon neutral fuel like biomass and it could actually be better. However I wonder about the actual total impact on global warming – the accumulation of heat trapped by that carbon starts * now* with the panels, and only some time in the future for the emissions due to fuel.

One small problem with this way of estimating fuel consumption is that the solar panel system is powering an air conditioner during the summer, the engine based system would not have any extra power available in the summer. So it’s not quite fair, but the air conditioner doesn’t run much so you could use a normal $250 (or $99 I have seen 2 stroke ones for) 1Kw internal combustion generator to power that on the few days it is needed.

http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=4558773 says ife of battry 2-4 years prob until 80 percent capacity

Battery capacity for the battery is sometimes stated in charge/discharge cycles, but this might not apply very well to photovoltaic systems

http://www8.tfe.umu.se/courses/energi/ExjobbCivIngET/Rapporter/Anton_A_06.pdf
http://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1137-99.pdf says we are talking about maybe 82% capacity after 2.5 years for the system but it is not an off grid household system, it is a streetlight like thing, that seems a bit high, have seen a document that I cannot find again now that indicated a loss of 52% over 3 years. Other documents indicate that the “lifetime” of the batteries is 2 to 4 years. However “lifetime” is not a very sensible way to measure it, because it is really about capacity loss, the “life” of a rechargeable battery (“secondary” battery) is usually the point at which it’s capacity is reduced to 70 or 80 percent of it’s original capacity, it can be measured in terms of charge discharge cycles, or time (given a certain usage pattern or ambient temperature or whatever). In some cases this makes some sense because the battery capacity drops off rapidly after this point, but it depends on how intelligent the battery charger is and how resistant the batteries are to overdischarging/overcharging and what kind of protection the batteries are afforded against these things by the charger and any other protection system (li-ion batteries always have complete protection systems built in as they are required to for safety).
Interestingly they only talk about the *battery* and never about *cells*….. hm.

So anyway, assuming 20% loss after 2.5 years or 8 percent per year seems reasonable for now.

http://photovoltaics.sandia.gov/docs/PDF/caploss.pdf VRLA are the type normally used
http://www.arcfinance.org/pdfs/news/EthiopiaPaper2009.pdf

http://www.harrisbattery.com/FAQ.aspx says vrla and sla are interchangeable (presumably AGM and gel cells are about the same or worse since judging from the the mechanisms of capacity loss seems like it would be only worse for these electrolyte situations)

some documents on battery life that look interesting but I couldn’t see because they cost too much:
http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=346940
http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TH1-48BKY5T-1&_user=10&_coverDate=07%2F01%2F2003&_rdoc=1&_fmt=high&_orig=search&_origin=search&_sort=d&_docanchor=&view=c&_searchStrId=1642936585&_rerunOrigin=scholar.google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=b4ec17f7c60014a0d8cc90d6b508a082&searchtype=a
http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TH1-3SDCDNP-2D&_user=10&_coverDate=08%2F31%2F1997&_rdoc=1&_fmt=high&_orig=search&_origin=search&_sort=d&_docanchor=&view=c&_searchStrId=1642935934&_rerunOrigin=scholar.google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=9618fe354c04c7ac20156f5939880d6a&searchtype=a
http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TH1-4F60NF9-5&_user=10&_coverDate=06%2F15%2F2005&_rdoc=1&_fmt=high&_orig=search&_origin=search&_sort=d&_docanchor=&view=c&_searchStrId=1642939054&_rerunOrigin=scholar.google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=f8ceca141ca8e41ae58b735bfe0435e2&searchtype=a
http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V50-3YYMR3R-Y&_user=10&_coverDate=10%2F31%2F1995&_rdoc=1&_fmt=high&_orig=search&_origin=search&_sort=d&_docanchor=&view=c&_searchStrId=1642938405&_rerunOrigin=scholar.google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=8a130b19325440b2822b9357335ce2d2&searchtype=a
http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TH1-4MM7TBH-2&_user=10&_coverDate=05%2F25%2F2007&_rdoc=1&_fmt=high&_orig=search&_origin=search&_sort=d&_docanchor=&view=c&_searchStrId=1642940091&_rerunOrigin=scholar.google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=64fc7e13574c94eb8062c42cb4fc63bc&searchtype=a
http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V50-4N6NPKY-2&_user=10&_coverDate=11%2F30%2F2007&_rdoc=1&_fmt=high&_orig=search&_origin=search&_sort=d&_docanchor=&view=c&_searchStrId=1642940341&_rerunOrigin=scholar.google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=08f5cc1f7bdfee291e31c92de38baa4b&searchtype=a

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Written by gregor

February 15, 2011 at 20:01

Posted in Uncategorized

4 Responses

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  1. thanks for another fine post Gregor…

    the tree saucers are currently intended to be equipped with 4 , 110 amp hour, 12v AGM lead acid batteries in series…the sealed formats are becoming better, and i need the safety margin they provide…

    i have, however,been keeping my eye on LiFePo (lithium iron phosphate) as this seems the most promising chemistry, in terms of competing with the lead acid, at some price point…

    this would allow me to switch to an approximately 180 lb.(82 kg) cast iron ballast insert for the keel, lowering the center of gravity for improved marine performance, while also increasing the capacity…

    rulgert

    rulgert

    February 16, 2011 at 10:56

  2. May I suggest tinyurl.com to make your links easier to use. 355 characters into this: http://tinyurl.com/4cvwb37

    et

    February 21, 2011 at 21:13

  3. To omuch like work though. Whadda ya take me for some kind of professinoal?

    gregortheinventor

    February 22, 2011 at 00:53

  4. […] which wear out fast enough that they have a substantial “running cost”, of roughly 8% the cost of the battery, per year, whereas NiFe do […]


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