Solar PV & ASHP might beat solar DHW

[DamonHD]

Hi,

I continue to flail around trying to find the best way to cover as much as possible of our primary energy costs at our small home in London, even in mid-winter.  (Roof and ground) space is a very significant limit to microgeneration here.

I have been looking at solar DHW, and of course one of the irksome things about it is that it provides little support in winter and if sized to get good spring/autumn support then it is throwing away lots of energy and stressing the system in summer.

While pondering ASHP (air-source heat-pump) here:

http://www.earth.org.uk/air-source-heat-pump.html

I discover that commercial R744 (eg Eco cute) ASHP units to cover space heating and DHW are achieving CoPs (coefficient of performance) of 5 or more, even in non-ideal circumstances, and even as much as a CoP of 4 with external air temps below zero and DHW of 45--55C.  (Installs of Eco cute systems in Japan have passed 1 million: http://www.r744.com/news/news_ida286.php )

I note that the most efficient retail solar PV on the market has an efficiency of roughly the reciprocal of that, ie ~20% for SunPower SPW-315 and the Sanyo HIT stuff I already have on my roof.

That means that a combination of PV and ASHP can end up extracting just about as much useful CH/DHW heat per square metre/yard/whatever from my available roofspace as plain solar thermal, albeit going though a helluva ride to get there.  It may even do better in cold air than plain solar thermal could.  And the maximum PV that we are allowed to connect without massive red-tape (3.7kW/16A) could make enough power on average to cover DHW demand even in December.

The nice thing about the PV/ASHP combo is, of course, no overheating in summer, and no 'rejected'/'wasted' energy, as anything that we don't use as electricity or via the heat-pump for heating can get pushed back into the grid (to displace some fossil-fuel burn elsewhere).

(GSHP would probably beat ASHP but we just don't have the ground area, and what we do have is riddled with sewers.)

Rgds

Damon

Shortened title to allow comments. woof

[wooferhound]

I googled DHW and found out that it means

Domestic Hot Water

[DamonHD]

Indeed.

Sorry, I should have more diligently expanded my abbreviations...  B^>

Rgds

Damon

[richhagen]

Interesting, It would of course be a much more costly install as well.  The PV panels are a multiple of the Water Heating panels, and of course the heat pump, exchangers and control system would probably cost more as well compared with the exchanger and control system for a hot water only system.  Still, the lack of wasted energy in the summer is intriguing.  I tend to agree that if ultimately these localized small systems are the methods that we use to power a significant portion of our homes in urban areas, we will have to take maximum advantage of what space we have.  What was GSHP, geothermal heat pump?  Rich

[scottsAI]

DamonHD,

Write up on Heat pumps:

http://en.wikipedia.org/wiki/Heat_pump

At -20C COP drops, still way better than past units!

Suggestion to put the air heat exchanger on the roof in the sun, with the sun shinning on it will heat it up some in the winter, summer cover it with a sun shield. Summer better to have on the ground, in the shade... Consider optimizing on which cost more, expect winter.

Evacuated tube Collector

These work great in cold areas, costly... considering your looking at ASHP is an option. Claim collect heat even in cloudy days.

If your roof is flat the solar heat collector can be designed to tilt to different angles over the year. Vertical collector reduces the heat collected in the summer preventing overheating, even tilt forward to stop heat collection.

Have fun,

Scott.

[DamonHD]

GSHP = Ground Source Heat Pump

At someone else's prompting, total cost about £28k for PV + £6k for ASHP as opposed to just £6k for solar DHW, but none of those figures is firm.

Rgds

Damon

[DamonHD]

Hi,

Yes, been reading the Wikip page thanks!

Oh, I'm not saying that this solution is the 'right' one, but it's interesting in terms of overall efficiency from a heat POV, and no overheating or 'wasted' energy.

Rgds

Damon

[powerbuoy]

I see it as follows:

My idea of a perfect mix is:

Thin film PV on the roof for the elec.-needs.

Accompanied by solar hot water for shower, cloth washer, dishwasher.

Geothermal heat pump for room heat.

That should cover almost 100% of the energy needs and might be somewhat affordable ...the geothermal might be pricy though.

PB

[Norm]

DTWSMAIJSTWDP

IMHO LOL

[DamonHD]

?

[Jon Miller]

Cloud nine eco houses use an ASHP and are only claiming a COP of 3 - 4.

GSHP claim a maximum of 6.

GSHP use the ground as the heat store, ASHP do not have a heat store/sink so I cant see how the ASHP can maintain COP 3 - 4.

If one has limited space, then solar PV and an ASHP would be a good combination, would be the most cost effective and be the most 'turn key' of any combination.

Of course, reduction in the form of less temperature difference and better insulation would be the first step.

food for thought.

[richhagen]

While it is difficult for me to estimate the depreciation on all of the equipment, it seems most likely to me that the additional costs compared with the value of the energy now wasted at even double the current costs of electricity would make it difficult to justify based upon cost alone.  A simple view:

 28000 - 6000 = 22000 = ballpark difference in cost

 22000 * .05 = 1100 = annual interest in British Pounds at 5%

if the maintenance and depreciation were identical it still seems unlikely that you would save even 550 Pounds on energy costs - or on difference in income on energy generated per year assuming a home energy use on a home about like mine.  $1100 U.S. is about 5500KWH at $.20 U.S. per KWH which is a bit less than double what I pay per additional KWH now.  

Another way of looking at it is that you would need a difference of about 15KWH average for every day of the year (at 20 pence per KWH) from the ASHP system to come close to competing at those costs.  This of course excludes any government or other incentives and of course assumes equivalent maintenance and depreciation.  

Still it is interesting to see how close to self sufficient one can come energy wise in an urban environment.  Rich

[DamonHD]

Hi,

Saving money is no part of the motivation.

Reducing carbon footprint, preferably to (say) current Chinese per-capita levels, is.

Now, there is probably a very large emboddied energy in the ASHP that I don't (yet) know about, but at least the refrigerant is sane in the models I'm interested in.

Rgds

Damon

[DamonHD]

Hi,

I see I used 'costs' misleadingly in my opening line: my apologies.

I meant costs in the CO2-costs sense.

Rgds

Damon

[BruceDownunder]

Damond,,, there should be an ENGLISH school near you .  I don't think they charge much ,,give it a go !!!

Bruce

[DamonHD]

Wouldn't have me!  Too disruptive, and speak wrong...

Rgds

Damon

[richhagen]

People like yourself and I and many who frequent this site may well do something like this anyway.  If it is to become mainstream, the costs will have to at least come close to making sense.  Most consumers probably will care most about these costs.  Rich

[DamonHD]

Yes, I'm sure you're right.

There's a decent prospect of the PV side getting much cheaper, soon, but I have no idea about the ASHP until I do more research.

Still, if 1 million+ of the ASHP variants that I'm interested in got installed in Japan that sounds like maybe 1%+ of households already, and some of these systems are as cheap as £500 here (maybe $500 in the US) and thus compete with ordinary boilers/furnaces.

So really, the silly bit is putting the RE on my own roof, and if you take that away you discover that lots and lots of people, especially in Scandinavia, where power is cheap and clean and heating is difficult/vital, have these heat pumps already.

So the foolish and expensive part really is the microgeneration, and maybe an understanding of the value of CoP, and trying to do it all in a house built to rather less good insulation standards than the typical target of a heat-pump install until now.

You think that'll deter me?  B^>

Rgds

Damon

[richhagen]

Well you have managed to get me to start looking at heat pumps here.  I currently dump excess to resistance heating in winter, but a heat pump through inverters should be able to replace more natural gas consumption much of the time.  Rich

[DamonHD]

Depending on what duty cycle you have that might be an expensive replacement for the resistive heating, and you'd have to be careful to avoid short-cycling the pumps.

But it might well be worth trying it with a cheaper unit.  At worst you get CoP of 1 and you're back to an expensive resistive load.

Would you use an air-to-air or air-to-water pump by preference?

Rgds

Damon

[richhagen]

Currently the main heating unit is a forced air natural gas unit.  There is an A-coil for air conditioning which I put in at the time and ran the lines out from the finished apartment.  I never installed the AC, but the refrigerant lines are still in place and should be appropriately sized for a heat pump as well.  I was looking at something like:  http://www.djsonline.com/amanaasz18_heatpumps.htm (but probably in a smaller 1.5 or 2 ton capacity.  

I am not an expert on these, but understand the basic concepts.  I basically would likely need to change out the A-coil and get a control that would work in conjunction with the natural gas heating and the available power source to optimize the efficiency of the heating.  Such controls have been commercially available here for such systems but I am just starting to look at this so much more research is in order.  I would likely need to be able to modify the control to take into account the state of the battery bank.

The main difference in an installation I would have would be the power source.  I would have to add a second inverter and connect both in a stacked lbx mode to power it.  I have one SW4048 connected to one phase now which works out OK, so I know that the concept is sound.  It would draw power from the renewable source when available, and from the grid when not.  I would leave the resistive dump load in place as  a failsafe.

Of course this would depend upon my ability to expand my rooftop solar arrangement.  I hope to add 6 more 190 Watt panels this summer.  Also, I will need to expand my storage.  I currently have a 440 Amp hour bank at 48V only.  

Like you my main goal would not be the economics, but the reduced consumption.  Rich