Zero Energy Case Study Report

Kevin_in_Denver_2

The whole point of net metering is that the utility company pays you retail for the excess PV power you put on the grid.

Rebates in CO pay for half the PV system, not much for the thermal system.

For DHW only, thermal makes way more sense. But a zero energy house already has a PV system by definition.

If the PUC adjusts the laws properly, you could purchase your DHW kwh's "off-peak" for $.03 and sell back kwh's during peak times for $.10-$.20

Kevin_in_Denver_2

PV can outshine solar thermal

The Habitat for Humanity Zero Energy home in Arvada, CO was carefully designed to produce as much energy as it consumes.

The latest report can be viewed at:
http://www.nrel.gov/docs/fy08osti/42339.pdf

The conclusion for this house, and this family, is that although the solar thermal did a good job, the solar thermal DHW system should have been omitted in favor of more photovoltaics and an electric resistance water heater.

If off-peak electric rates are available, it makes the decision even easier, because off-peak electric btus are now cheaper than natural gas btus.

Granted, Denver is quite sunny, but this house just wouldn't cost much more to build than a builder tract house, and it's actually producing way more energy than it's using. That raises the point that the net metering rate structure should be changed to allow net zero homeowners to get monthly refund checks from the utility company when the design overshoots.

jp_2

hard sell

I have a hard time buying this one.

PV's for DWH more efficient than solar hot water?

using a 20 gallon heat would require(1500W) about $8,000 just in PV panels out of the box. and this would give you that only for maybe 6 hours?

Metro Man

not buying it...

Solar DHW sized @ 96 sq ft w/ 200 gallon tank. At optimum angle( 45*) should be 144 gallon tank. Collectors are @ 27* about 1/2 the optmum angle.

Need to read through all but that's what jumped out at 1st look.


Metro Man

Chas_4

Subsidies

You forgot the Government cheese. I can get 5KW for 8K here.

Kevin_in_Denver_2

PV vs. Thermal Shootout

Also, this family has lower than expected hot water usage. As you know, if you don't use the collected energy, you essentially lose it because the solar stops collecting at high temperatures. Therefore, smaller than design loads can really hurt overall system efficiency.

PV has the advantage in this case because the collected energy can at least be fed back on the grid and actually be used by someone else. Another advantage, which is huge in my view, is the simplicity of the system. The mean time between failures is much longer for PV, and it's easier for the average homeowner to tell if his PV system isn't working.

Some brainstorming here: what if you could send the excess thermal heat to a Stirling cycle genset and send electric power back to the grid? The theoretical efficiency of that cycle is actually better than PV.

Metro Man

failure

Still haven't had time to read through report but curious of the "mean time between failures" statement. Solar thermal system we design and install will chug along for many years without any issues.

PV panels and thermal panels will both last indefinitely. We have systems of both 25 years and running great. Weak link for the PV system will be the inverter. Weak link for thermal is storage tank. That's why we install stainless steel non-pressurized tanks. Inverters have come a long way in the last 5 years and are proving much more reliable. Lightning is probably the biggest worry for Joe-homeonwer.

Efficiency wise - no comparison. Solar thermal flat plates come in @ around 55 - 70% efficiency. PV is currently 15 - 16% with some hybrids @ 20% but $$$

It's too bad solar thermal got left out from Colorado's amendment 37. It would have been easy to meter btu's generated by a system. We could have been providing customers with gas rebates along with the electric rebates. If I could only figure how to pump btu's into the gas line.........

And for customers who do not have the advantage to take the Xcel rebates..... not many PV systems going in there but you will see solar thermal.

Metro Man

Kevin_in_Denver_2

MTBF

I grant you that drainback systems are extremely reliable (I designed them in the early 80's), however, there are several things that can (and will eventually) fail:

1. Collector pump-expected max lifetime-30yrs? A very small percentage of them fail in 1-10 yrs.
2. Differential controller-failure rate >0%
3. At least two thermistor sensors - failure rate >0%
4. Tank evaporation gone unnoticed by the next owner of the house, because he wasn't briefed on the system.

A PV system has the inverter only, which is dead easy to see if it's working or not.

My point is that all new homes should be designed to be zero energy. This Habitat home proves that it's not expensive to do.

A zero energy home MUST create electricity, and PV is the current state of the art. Why introduce a separate thermal system (in this case $7,000) which adds overall complexity when you can spend that $7k on more PV.

I'm looking at it from the new homebuilder's perspective, and of course we always want to reduce headaches. More PV is less of a headache than the additional mechanical system.

Mike T., Swampeast MO

Thanks Kevin! A very interesting read.

DHW consumption by that family seems to be extremely low for that year at least. As the study suggests, it's extremely difficult if not impossible to predict the habits of occupants. Sounds like a mother and two younger children. I have a feeling that DHW consumption will increase significantly as the kids get older.

It would also appear that the kitchen range is not used very much. As best as I can tell, it averaged about 6 kwh/hr per month or only about 8 minutes of a 1,500 watt burner on "high" per day. Not much cooking usually means there's not much dishwashing, so this could account for a big chunk of unexpectedly low DHW use.

jp_2

who foots that bill?

ok, someone gets cheap PV panels, we all pay for that!

Kevin_in_Denver_2

Cheaper PV coming

Yeah, I know they've promised it for 30 years now, but $1/watt
PV is supposedly coming soon. That really should tip the scales for most of us.

AVAsolar in Fort Collins is ramping up production.

jp_2

apples to oranges

I do agree this is the right direct.

BUT.... one study done with abnormally low hot water usages doesn't cut it. most all posts here talk about nothing less the a 60 gallon indirect for hot water.

Is using subsidies really valid for proving system effectiveness?

in a perfect world every roof top would have PV's and we would rely on wind energy at night.......no power stations......

why spend all this money if most your power goes down the street during the day when you are at work, then end up buying power after sundown? why don't you just give your neighbors money directly?

if you store the energy you produce, you do not need such large panels? more economonical I think.

what would the true cost of a PV hot water system cost compared to a collector system for a TYPICAL family????
can't build systems that work in only special situtations?

Kevin_in_Denver_2

oops

double post

Unknown

as opposed to, someone fills up their gas tank, and we all pay for it (via our army)?

jp_2

gee rob

you are right, if we subsidize alcohol enough, we can make our own energy..........

I like TACO's bottom line statement, "DO it right, do it once"

jp_2

when is peak

times?

still ask, do subsidies trump practicality?

reminds me the the alcohol fuel subsidies.

by the way, michigan has no such program.

can't see a couple bucks back a day worth it.

best would be for large amounts of people going off the grid, then power companies would be hurting and offering incentives, passing laws ain't a real good idea, you may be next! care to work for minimum wage?

Metro Man

Flawed

I would really have liked to have had an opportunity to consult on this one. I would have installed (for testing purposes) a radiant heated in - slab hydronic system with an electric back up boiler utilizing solar thermal heat with sdhw. Slap on a Wilo (if we can ever get them) or similar variable speed and you reduce electrical consumption even more.

If you going to use state of the are PV components it would make sense to do the same thing on the thermal and heat side. Electric baseboard heat, gas heater, these are archaic by today's standards. A larger storage tank, correct angle tilt and a little longer solar circ run time would produce a considerable amount of Btu's.

With the super insulated home (15,000 btu's / hr demand) a solar thermal system along with a the PV would have been a good match.

The flaw in this solar dhw design is the solar collector array to solar storage tank ratio. With the angle of the thermal collector array I'm getting about 120 gallons of storage @ 140*F. The solar pump would have run much less throughout the year considerably lowering the annual electrical consumption.

Installing $34,000 worth of PV isn't the way to reduce a home's energy consumption. Not all states have the luxury to utilize the PV rebate's Colorado now enjoys. Using the best available technology for today new construction in homes is the way to go. Appliances, doors and windows, insulation, lighting, and heating all should go hand in hand to create homes with low carbon footprints.

But.... I applaud NREL and like organizations to put live testing like this for data extrapolation. We need more of this. Maybe I'll get to work on the next one.



Metro Man

realolman

in PA

we pay .096 / KWH... in Morgantown WV my son pays .062 after taxes, and any and all other charges. I just looked at both bills.

Total bill / KWH = .096 PA. .062 W.V.

I can't see the power co. paying more to buy juice than it sells it for, or selling it for less than it costs.

EDIT: After reading the article I see that the TVA buys power for twice what it sells it for... I can't figure that one.

I saw another post calculating 8 min of range use per day. Believe me, I'd love to see some sort of energy alternative, but it has to be realistic.

SpeyFitter

No I have not read the report> Not Yet BUT From my 17 years experiance ,Solar thermal Has no competition with PVs' @ 12-16% eff if that due to low light production. ...Paul

realolman

I didn't see

what it cost to build the place. The square footage is on the inside or the outside dimensions? the place doesn't look that large on the outside, judging by the door width.

Abro

Power Pricing

"EDIT: After reading the article I see that the TVA buys power for twice what it sells it for... I can't figure that one."

One nice thing about PV is that it produces peak peak during peak loads. In Ontario you can actually set up small PV ops and sell the power for about 40 cents per kWh. The demand pricing at that time of the day is what helps justify it.

The problem with electric pricing in most places is that it should have several prices depending on time of day. Overnight power is dirt cheap, and the PV isn't offsetting anything then.

realolman

well

thank you for that. I guess that does make some sense. I understand a little now.

Kevin_in_Denver_2

Habitat House Costs

The costs for H4H homes are always extremely unrealistic because of all the volunteer labor.

But they are always built simply. This house would cost most builders in the area only about $100/ft2 before all the solar stuff.

The "extras" were $7k for Dhw solar thermal, approx. $10k for PV ($22k) without rebates. HRV was roughly $2k. Note there was no conventional heating system, which saved about $3k.

The "simple" construction was why the thermal panels were on a low tilt. $300 in legs could have tilted them up properly, but the NREL guys believed its not an issue on a typical DHW load.

Metro Man

$$$

So the numbers should be;

$22,000 (div) 4000 watt PV = $5.5/watt. Assuming materials only?

$7000 (-) $2000 (fed tax credit) = $5000 (-) Assuming labor + materials included?

$7K sounds about right as a TOTAL for a sdhw.

If volunteer labor (would like to get me some of that) was used for PV and not for thermal and tax credit not taken out and then final #'s are tallied for comparison....... Houston we have a problem.


Metro Man

scott markle_2

use it or loose it

The conclusions of this study are unfortunate in that it has the potential direct more investment in P.V over thermal systems. This is already a imbalanced situation with thermal systems receiving much less attention than deserved. We know that a thermal collector has almost 5 times the potential efficiency of P.V. but this study shows an actual annual utilization efficiency of only 4.8% ! what went wrong? I believe It's a use it or loose it issue.

When the cost of pumping is factored in it's not hard to agree with the conclusion that this particular system is not working so well. The unfortunate part is that instead of asking how can we utilize solar thermal more efficiently the authors seem to be concluding that an all electric approach might be favorable.

The dilemma for efficient solar utilization will always be storage. With Net metering every P.V watt collected gets counted, is this completely fair? does every electron put back in the grid end up doing work? If every suitable roof in america had a P.V array could the grid properly regulate this decentralized generation? If we could generate all our electricity with P.V what would we do at night? In reality net metering can only work if this form of generation remains a fraction of our electric mix.

The all-electric zero energy house is no environmental panacea because It exists only with the support of conventional electrical infrastructure. It reminds me a bit of the contradictions inherent in carbon trading. How fundamentally progressive is the ZEH if it takes nukes and coal to make it work?

As far as 4.8% efficiencies. Let's assume 5kw/sq.M/day X 9 meters X 360 days X .048 = 777.6 kw/year Lets say 20 years of operation. 20 X 777.6= 15,552 kw/ 20years. cost of system $7000/15,552kw = .45 - 45 cents a kw/hr as the 20 life cycle cost for solar DHW. Not very exciting numbers,(I think we can do much better) but from what I understand fairly consistent with unsubsidized P.V life cycle costs. Did I get that right?

Kevin_in_Denver_2

PV vs. Thermal costs

I think this is a better look at the relative costs, and it comes directly from the report:

“What would it cost to increase the size of the PV system and use an electric tank heater?” Conventional wisdom hold that the solar water heating system is a better investment. A comparison of the two systems is shown in Table 7."

Table 7 goes on to show that the thermal system with gas backup cost $9289 vs. $5102 for PV/electric water heating.

This is truly of a surprising result for those of us with 17 years (or more) in the industry. Historically, PV never could compete in DHW.

However, as a homebuilder, I'm happy with the results because it means the ZEH's I build will be technically simpler.

Kevin_in_Denver_2

Germany

In Germany, they determined that it is their future best interests to pay FIVE times as much for PV generated electricity. That is what caused the recent shortage of silicon and kept PV raw materials high.

A little googling will explain why they did such a crazy sounding thing.

Now that more silicon production plants are coming on line, the cost of PV will continue to decline. Remember, silicon comes from sand. On the other hand, I don't see copper prices coming down to help the costs of flat plate solar collectors.

scott markle_2

But the thermal system in the report only operated a 4.8% annual average utilization efficiency. interestingly it's efficiency was highest in december.

scott markle_2

Kevin, I think your jumping the gun on this. Simplicity has tremendous importance and merit. But I think it would be a big step backwards to use this report as a justification for abandoning solar thermal, in favor of electrical resistance and P.V. Even if subsidies and metering structure make it economically favorable. The germans have progressive energy policy and supporting the higher than conventional cost of P.V generation is part of that. I't makes plenty of sense to supplement the gird during peek hours with P.V and metering laws should bolster this approach. However I don't think you will find a lot of electrical resistance heating in germany, I wouldn't be surprised if it was illegal there regardless of ones net electrical usage/production. I don't think you will see germans abandoning solar thermal anytime soon.

And lets be real about the whole notion of ZEH. While sunshine may be free it can take plenty of energy/money to collect this "free" energy. People glom on to these sort of buzz words to sell the next big thing and stop thinking critically about the contradictions. Slapping a bunch of silicon on a super insulated house does not profoundly diminish the fact that housing has a huge energy cost, from the trees no longer fixing carbon in a forest, to the to the sand "miraculously" made into a photo-cell. This zero thing insults the Sun, zero energy.. does it have to come metered from a utility to count.

Kevin_in_Denver_2

ZEH is a worthy goal

One last point about this report. When your home is approaching zero energy, the fixed monthly hook-up charges become significant. If you have gas backup but don't use any gas, you still pay $12/month. You can't really avoid the $7/month electric fixed cost without a bank of high-maintenance batteries. This fixed cost problem is just one more reason to use electric backup.

Scott,

The main point of this house is that it doesn't cost much to collect the "free" energy. Again from the report: "It is possible to make efficient affordable ZEHs with standard construction techniques and off-the-shelf equipment."

People need housing. Building housing does have a huge energy and environmental cost. However, I think that slapping a bunch of silicon on a superinsulated house is the best thing we can do. If there are better ideas, let us know. New housing need not be built on farmland, we need to place it in the existing cities, where the infrastructure already exists and many of the existing homes are functionally obsolete.

Kevin_in_Denver_2

European ban on Electric for space heat

Denmark has recently lifted the ban on electricity for heat in new low energy buildings. The primary reason for this is that in the future, utility-supplied electric will receive a greater and greater share of renewable sources, while natural gas obviously will not. Denmark has probably already reached that tipping point as they are one of the largest producers of wind-based electricity and wind generation equipment in the world.

http://ec.europa.eu/energy/demand/legislation/doc/neeap/denmark_en.pdf

jp_2

I'm all for it!

if you can generate enough electricity, via sun,wind,water or other renewable sources.

Metro Man

Energy Efficiency

Since this thread is still active and I'm waiting for an email I thought I'd add another 2.5c

I still believe that when building homes using the best technology in all categories should be taken into consideration. As I mentioned b4 the thermal space heating and domestic hot water heating seemed to be more of an after thought to the design of this federally funded project. Had a low temperature radiant heating system been employed, more thought to the domestic hot water loads, and proper sizing the results would have been different.

I don't think this technology would have added to much complexity to the home since this is tried and true, state of the art, field tested components. Not to mention the comfort factor or even potential for solar cooling.

ZEH's start with super insulation, I am all for that. Interfacing the various renewable energy sources to these homes is what we should be exploring and spending our research money on. Concentrate on lowering the loads for buildings and not just matching a load to the flavor of the month rebate, or tax credits. These are temp incentives.

Since working with radiant heating systems for the last 20 ish years I don't see why given the opportunity anyone would install anything else. The comfort, low temps, and zoning capabilities are second to none. Electrical loads for these system are also very low and continue to lower with new pumps and controls.

Emails in gotta get back to work....

Metro Man

realolman

where

does some one learn about this stuff... the solar heating and all. I am very interested, but always figured that living on a rocky crag in PA precluded any solar anything.

I imagine denver is probably not too hot either.

Metro Man

Solar fever

Denver gets 300 + sunny days per year compared to 189 (I think) in Pittsburgh area (where I'm originally from 28 years ago). Perfect for solar heating because of the heating loads vrs. sunny days.

I haven't seen any "how to do it" books that I thought where that great. But if your good at designing heating systems your 1/2 way there. BTU's are btu's. NABCEP, SRCC, SEIA, NREL, and reading what you can get yours hands would help. If your interested in something specific you can email me directly. Oh yea the "Home Power" magazine is good.

Metro Man

jp_2

get youself

a Real Goods catalog

Real Goods

they have some interesting books

scott markle_2

south facing rocky crag?

Viessmann has really good supporting literature. They have a System Design Guideline(draft version) that is excellent. It does not seem to be part of the online literature though.

scott markle_2

Kevin, I appreciate you posting this sort of stuff but I found this article a bit obtuse and hard to digest. The distinction between socio-economic and private economy potential for example, I just don't really get it. I think sometimes this policy stuff gets away from it self and common sense starts getting hard to understand.

The one thing I am getting is that for better or worse economics will profoundly influence the alternative energy industry. Interestingly it's getting to the point in some parts of the country where electric resistance is a fully competitive form of heating. This is not good, as nearly half of this countries electricity comes from coal. How do you keep markets free but prevent opportunistic forces from leading us in environmentally harmful directions?

Again I really don't believe that an all electric house, (no mater how much silicon it has connected )is the path we should be following. Lets face it the watts used for nighttime heating (coldest temperatures and no passive gains) are coming largely from coal, your daytime solar generation can't fundamentally change that, you may have a net zero electric bill but you couldn't do it without coal. It's like flying off to Hawaii and paying someone to plant a tree in the amazon to "offset" the carbon. Yes this may be a net zero equation (carbon wise) but strikes me as being a bit to convenient and dishonest in a way. The difference with collecting solar thermal energy is that its almost totally independent (manufacturing not included)of this dirty partner.

Kevin_in_Denver_2

Builders should react to energy costs

Economics, like it or not, are the driving force behind builder's decisions. This Habitat house is a great example of a zero energy home that any builder can handle. As a bonus, it obeys KISS principles.

Builders shouldn't be expected to know the best mix of energy sources for a new home. They should only react to the existing rate structures even if it seems dishonest. Net zero yearly energy cost is the goal, and will soon be a market requirement if not a legal requirement.

In Colorado, the PUC can tweak rate structures to encourage the best mix of electricity supplies. Xcel is bringing wind energy on line as fast as possible, even faster than Colorado voters mandated thru Amendment 37.

Electrical demand side management can easily eliminate the need for new coal generation plants. This should be done ASAP. (Think off-peak rates) Then, as renewables slowly replace the old coal plants, the CO2 problem gets solved.

I see a future where PV fuels the daytime, wind and hydro fuel the night. The rate structure only serves to optimize the mix.

Natural gas is then useful to generate electricity only during rare spikes in peak demand. In our future of renewable electricity, gas has no business heating homes or even cooking food. AFAIK, no one is even working on "renewable natural gas". The portable fuel of choice will likely be hydrogen since no CO2 is produced when it's burned.

Unknown

Renewable gas

http://www.technologyreview.com/Biztech/19128/

jp_2

gotta disagree scott

all electric house would best of all worlds.

zero emissions from every household fully electric!

add wind,water and solar power, zero emissions, and of course, you need to store your days power generation so you can use it at night, better batteries are not far away.

yes, we need to walk away from coal.