Has anyone installed a Velux roof integrated solar thermal system?

[SolarAndWood]

Saw it on This Old House this afternoon and like the way it looks. Does anyone have one?

(broken link removed)

 

[GaryGary]

Hi,
I don't have one, but that's a pretty clever approach to getting a collector through an HOA -- very nice look.

Here is the SRCC rating page for the large one:
(broken link removed to http://securedb.fsec.ucf.edu/srcc/coll_detail?srcc_id=2007018A)


This the SRCC rating for a Heliodyne collector of nearly the same size:
(broken link removed to http://securedb.fsec.ucf.edu/srcc/coll_detail?srcc_id=2010115C)

They are quite close in performance, and the Heliodyne is a very good collector, so that's good.

Gary

 

[SolarAndWood]

Thanks Gary. Seems like nice leverage of the experience they have putting holes in roofs and a real nice job with the aesthetics. I ice and watered my entire roof so I don't have any real concerns with their install.

 

[begreen]

Velux makes great skylights. We have a few and they have been totally trouble-free.

 

[woodgeek]

I got a VELUX flush-mount system quoted out a couple months back, figuring with my 70% (PA+FED) it would be no brainer. My installer wanted $13k for a two panel system and $15k for a three panel. After
rebate that is $3-4k for me.

I decided against it after looking at my monthly insolation numbers from this website: (broken link removed)

Looks like my solar resource is good from a hours of peak output perspective (>1000 hrs/yr), but is basically hopeless in the winter months, due to clouds. When I looked at the Velux system, the storage is pretty meager b/c the tank always needs to have some hot water for a call, but also needs to store extra when solar input is high. Basically, the backup coil kicks on when the top half of the tank starts to go cold, and the system dumps excess heat (circulating at night) when the bottom half is too hot. Bottom line, the tank is half as big as the nameplate value for delivery (FHR is low), and the storage is half what you'd expect from the full volume swinging. AS it is, it stores about one day output, so your morning showers came in yesterday afternoon. If yesterday was cloudy, you are showering on electric backup heat.

So, I convinced myself between the winter months and lack of cloudy day storage, I would be unlikely to get more than 60% of my heat from the solar, the rest from electric backup. For a similar amount of out of pocket money, I can get a ASHP water heater, that will use 50% of a conventional heater (within spitting distance of the 40+% solar backup usage), get better FHR, and get a lower maintenance and more compact system. I have the perfect place for the unit, a large, semi-conditioned attached 2-car garage right next to my plumbing stack. YMMV.

So, I would love to have folks tell me I'm wrong. I'm a fan of solar and thought solar thermal would be a clear winner. But those damned refrigeration engineers are making 'free' solar BTUs look expensive.

If eff rather than $$ was the object, I would get the Velux system, leave the backup coil disconnected (getting >2x the storage) and feed it into a 50 gal geospring (to get eff backup) and then to the taps.

In 10 years all the solar tanks might have integrated HP backup....maybe I'll get one then if elec is 2-3x as expensive as now.

 

[GaryGary]

I got a VELUX flush-mount system quoted out a couple months back, figuring with my 70% (PA+FED) it would be no brainer. My installer wanted $13k for a two panel system and $15k for a three panel. After
rebate that is $3-4k for me.

I decided against it after looking at my monthly insolation numbers from this website: (broken link removed)

Looks like my solar resource is good from a hours of peak output perspective (>1000 hrs/yr), but is basically hopeless in the winter months, due to clouds. When I looked at the Velux system, the storage is pretty meager b/c the tank always needs to have some hot water for a call, but also needs to store extra when solar input is high. Basically, the backup coil kicks on when the top half of the tank starts to go cold, and the system dumps excess heat (circulating at night) when the bottom half is too hot. Bottom line, the tank is half as big as the nameplate value for delivery (FHR is low), and the storage is half what you'd expect from the full volume swinging. AS it is, it stores about one day output, so your morning showers came in yesterday afternoon. If yesterday was cloudy, you are showering on electric backup heat.

So, I convinced myself between the winter months and lack of cloudy day storage, I would be unlikely to get more than 60% of my heat from the solar, the rest from electric backup. For a similar amount of out of pocket money, I can get a ASHP water heater, that will use 50% of a conventional heater (within spitting distance of the 40+% solar backup usage), get better FHR, and get a lower maintenance and more compact system. I have the perfect place for the unit, a large, semi-conditioned attached 2-car garage right next to my plumbing stack. YMMV.

So, I would love to have folks tell me I'm wrong. I'm a fan of solar and thought solar thermal would be a clear winner. But those damned refrigeration engineers are making 'free' solar BTUs look expensive.

If eff rather than $$ was the object, I would get the Velux system, leave the backup coil disconnected (getting >2x the storage) and feed it into a 50 gal geospring (to get eff backup) and then to the taps.

In 10 years all the solar tanks might have integrated HP backup....maybe I'll get one then if elec is 2-3x as expensive as now.

Hi,
Those prices are pretty steep even for a commercially installed system, but maybe the Velux equipment is more expensive.
A commonly used number for commercial solar DHW systems is about $8000 -- which still seems very very high to me. We have a local guy here who does good systems for about $5000.
On the other hand, your rebates are really good.

As you likely know, for high solar fraction on solar hot water, its good to oversize the collector a bit from the usual rules and tilt it steeper and carry somewhat more storage than the usual rules. Systems done this way can provide solar fractions up in the 0.9 area -- ours does a measured 95% year long. But, with the Velux system (nice as it is in some ways), you are kind of stuck with the current tilt of your roof, and perhaps with the tank they offer -- although I don't see any technical reason you could not use the Velux collectors with any tank/pump/controller system?

Solar water heating is one area where building the system yourself really pays off. The system I use was $1000 in materials, and provides 95% of our hot water (we are not big time hot water users). With the MT $500 rebate, the out of pocket cost was $500.
http://www.builditsolar.com/Experimental/PEXColDHW/Overview.htm
I've since upgraded the collector area to do some space heating as well as solar hot water -- total system cost with 100SF of collector and space heating additions was $2000:
http://www.builditsolar.com/Projects/SpaceHeating/DHWplusSpace/Main.htm
These are drain back systems with non-pressurized storage tanks -- while I guess a yearly check on how things are going is always a good idea, there is really not much of anything to maintain on the system for the first 10 years or so. There is no antifreeze to go bad.

The first system actually puts out almost exactly the same amount of energy per year as my $10,000 PV system! ($6500 after rebates).

So, if you like to build stuff, you might consider that. I think that one could actually DIY a collector with the same flashing approach as Velux uses and get very nearly the same look.

I think the heat pump water heaters can make sense. One thing to bear in mind is that the heat pump will cool the air in the area its installed. That's nice in the summer, but in the winter, it means that if the HP is in conditioned space, your furnace has to reheat that cooled air. For us with an 8 month heating season, that's a drawback. From what I have heard, maintenance and durability can be an issue, so get a good one.

This is a little section on HP water heaters:
http://www.builditsolar.com/Projects/WaterHeating/water_heating.htm#HeatPumpWater Heater
The last paper has a partial solution for the bit about the HP adding to the furnace load in the winter.


Gary

 

[woodsmaster]

I've installed a lot of velux sky lights and had no problems.

 

[SolarAndWood]

Thanks guys, lots to think about.

My biggest challenge is the solar south roof was optimized for annual PV production because of our net metering arrangement in NY. So, the South side is 7/12 and the North is 16/12. It would seem I put the trusses up backwards. I am thinking I can extend the 16/12 plane up high enough to get a row of collectors across the top on the south side at the steeper angle. Not sure if I am any better off with that than just putting a mounting rack on the roof.

 

[woodgeek]

Gary,

I am a big fan of your website and I agree with everything you are saying. I've got a 4/12 roof facing Southeast, and maybe 20% tree shading to boot. You are correct that for a steeper roof angle and no shading, I might expect much better than 60%.

The Velux tank/controller is def cadillac--I did not find out who really built it. Based on Euro prices, I expected prices more in line with what you suggested--it is clear there is an extra markup whenever the incentives go up, as in PA. That said, I was still disappointed by the FHR and the BTU storage, since both were based essentially on a half tank volume. I think the low storage will hurt overall utilization (storing one days usage is not enough). So, to get close to 90% I would need a secondary conventional tank, but I suppose it needn't be HP since it is only doing 10% of the load. Of course, I could also drive up the percentage by restricting my hot water usage hard, not showering after cloudy days, etc, but my fam is not going to go for that.

I fear that many folks get these systems installed without a real thought to actual energy savings, and get a feel good vibe with systems that are running 50% backup. And the installers do not help this issue--they would rather have more (invisible) backup than a dissatisfied customer running a higher percentage (but getting a lower FHR).

Oh yeah, my 50% ASHP figure includes estimates of local cooling and heat draw from the house. The actual COP in HP mode is more like 2.4, and my calcs suggested that in the dead of winter I would have to run some resistance heat (when the garage goes below 50°F), puling my SCOP down to ~2.0 or so.

 

[SolarAndWood]

I would be happy with 50% backup if the price was right. My thinking has been that a 3 panel system on the 7/12 solar south unshaded roof would perfectly compliment a wood boiler with modest storage. Then, when the cost of PV comes in line, the rest of that roof generates electricity.

 

[woodgeek]

I would be happy with 50% backup if the price was right. My thinking has been that a 3 panel system on the 7/12 solar south unshaded roof would perfectly compliment a wood boiler with modest storage. Then, when the cost of PV comes in line, the rest of that roof generates electricity.

Seems you can use the PVWatts tool (link above) for your roof and location to figure out your expected monthly BTU output. I entered a 1 kW 'system' (1 kW at normal incidence in max sun), and then the kWh per month reported by PVWatts are my max output hours per month. Multiplied by the estimated max output BTU/h of the Velux collector gives me my expected BTUs per month. You can then look at that figure relative to your typical daily usage to estimate your _minimum_ backup requirements. Actual backup might be higher if the storage overfills some days and bottoms out on others.

Apologies if this is obvious to you already--I found the exercise helpful.

OF course, with a wood boiler backup, esp if already installed, the backup operating cost is less of a concern. For me the installation cost of an ASHP and three panel system, after incentives, came out the same, and the operating cost (i.e. the amount of backup elec), were within $50/yr of each other. So I will be iopting for the 'simpler' system. My 4/12 SW facing roof is not so bad for PV as for thermal, and I would also be mad if the thermal panels 'got in the way' if/when I go PV in the future.