DHW indecision

[Stones]

The trouble with constantly reading about different ideas, is that you begin to question decisions that you have made, and in my case, the thing I'm currently questioning is DHW provision. 

I had initially planned to have a 300L UVC in our utility room to service all our DHW needs, heated by an ASHP then topped up via immersion or an inline heater.  When I sat down and worked out the pipe runs involved in getting DHW to my bathrooms, I concluded that it wasn't the most sensible location for our cylinder.

Having made the decision to locate the main DHW store to a cupboard in our en-suite, which significantly reduces pipe runs, pipe losses and time taken to deliver DHW to the taps, I began to focus on a 4 cell SunampPV as the DHW store of choice.  After a bit of back and forth, Sunamp confirmed they could supply a 2 cell unit and an additional 2 cell booster pack to give a 4 cell capacity.  A 4 cell Sunamp would give me the capacity I need, and eliminate 80% of our anticipated cylinder losses, which over the course of a year adds up to a fairly tidy sum.  I'm just waiting to clarify a couple of points in respect of the price they have given me (more than a single unit, less than 2 units at their current buy one get one half price offer), but needless to say, I'm wondering again, whether I'd be better off going with a straightforward set up of 300L Direct UVC for DHW, with a smaller 15L point of use DHW heater for the kitchen and utility, and spend the rest of the capital on a 1.5kWp or 2kWp ground mounted solar PV array.  

Cylinder losses in winter would be useful in terms of heating requirement, however in summer, of no real benefit. My heat loss calculations (which include incidental and solar gains) suggest that with summer bypass on the MVHR, I shouldn't have any significant overheating issues - I do have the 'luxury' of fresh summer breezes that I can take advantage of by simply opening a window to help cool the house down if required.

Solar PV would offset the wasted summer cylinder losses and still leave a healthy amount of generated electricity that could be used in the house, perhaps saving £100 a year of my electricity bill with any excess diverted to DHW, saving another £30 or so.

I'd be interested to hear thoughts on my current thinking.

 

 

[Mikey_1980]

I can completely sympathise with you having gone through this 12 months ago when the sun amp was't even around to make it more difficult. We went through the whole Oil/LPG/ASHP/Eletric/UVC/Thermal store debate lots of times with lots of people with lots of different answers. In the end I found a comoany that I was comfortable with, seemed to know what they were talking about and listened to what we wanted and so far it is working well for us, but when buidling a low energy demand house but wanting to have plentiful DHW it is a difficult conundrum.

[SteamyTea]

Tricky one.

Cylinder losses are uncontrolled loses, so not that useful.  And they are probably larger than you think.

Really comes down to the basic arithmetic in the end:

Reasonable Expectation of Usage + Acceptable Losses = Real Usage

Price of Different Technologies / Longevity + Running Price = Real Unit Price

 

For a laugh I have just looked at how much water my kitchen sink holds (can't believe I have never measured it before).  It is 12 lt or 12 kg.

My incoming water temp at the moment is 18°C (at the moment) and my DHW temp is 50°C (I am totally reckless and never worry about water disease).

So that is 0.45 kWh

My bath holds about 100 kg of water, but is at 42°C so that is 2.8 kWh.

My shower uses about 30 kg at 38°C, so 0.42 kWh

So my daily usage is about 4 kWh/day during the summer.

Because I am on E7, I have no real choice but to use a 'thermal store'.  Mine is a simple 200 lt copper cylinder that was made in 1987 and had little insulation around it.  My losses where around 4 to 5 kWh/day, but I have reduced this to 1 or 2 kWh/day with extra insulation.  It varies with the seasons (mains temp can go as low as 6°C) and usage (if I use less, I have greater losses, so bath first thing, shower later).

So from my simple usage I can work out what is the best financial solution.

Halving my standing losses saved me around 20p/day or £73/year.  This easily paid for the extra insulation as it was only 2 sheets of Celotex that cost £50 delivered.  I am still upset that I have losses, but the alternative is to fit an inline heater and run it at full price electricity, so that would realistically cost me about £700 to fit and near enough double the running costs.  So not really worth it.  Same with the Sunamp (which has excellent insulation and very low loses).

Now as you don't have to rip out any old system, get cabling relocated, you have an advantage over me.  You need to look at the balance between amortised installation price, losses and running cost.

I suspect that there is not much difference in the cost of a Sunamp and a new decent thermal store.  Both will realistically need an inline heater as you will have times when your DHW usage is high (visitors).  I have no idea how long the pipe run is to the kitchen, but you may find that a dedicated inline heater is best, it may reduce the total plumbing as well (just needing a single cold feed).  But you may find that you can live with the delay and just fill the sink with the hot tap till it is hot enough (mine is about 2/3^rd full, so about 8kg of water).  Up to you to decide on that one, you may have a dishwasher anyway.

 

So, sit down, do the arithmatic and the answer will fall out at the end.

"With a little physics, you can do anything Jim" Hazel Rymer, Life Scientific, Radio 4

 

 

 

[Guest Alphonsox]

I'm still of the opinion that the Sunamp ticks most of the boxes. Although potentially a more expensive solution the control of losses and small size make sense to me. In our case the main decision is whether it makes sense to install PV or to use E7. If I had access to E10 electricity the decision would be made.

[SteamyTea]

43 minutes ago, Alphonsox said:

If I had access to E10 electricity the decision would be made

If the power industry every sorts itself out with smart metering, then that will make life a lot easier.

[PeterW]

37 minutes ago, SteamyTea said:

If the power industry every sorts itself out with smart metering, then that will make life a lot easier.

I can pretty much categorically say it will not be to the benefit of the consumer !!!

The technology is fraught with issues (it was built to the lowest price contract...) and its being rolled out against a set of standards that aren't fully agreed....

[ragg987]

I also specified a 300l cylinder based system, then looked at Sunamp quite late in the day, but decided (with input from Sunamp) not to go with the Sunamp. Also, the cost / benefit is marginal. The 4-cell Sunamp costs £1,000 more than a 300l cylinder. If the difference in standing losses between Sunamp and cylinder is 1kWh per day, would take over 20 years to break even (simple straight line calculation without the cost of the capital included - would be longer than 20 years if included).

If you have a separate cupboard for the cylinder, why not lag it all round so that heat loss is reduced? After all, the primary difference between the Sunamp and a cylinder (from heat loss perspective) is quality of insulation. I intend giving this a go.

One other factor, if you intend to use a ASHP to fulfill your DHW needs, you will need a high temperature unit to active the PCM in the Sunamp (I think Andrew said >60 deg C). This is going to hit your COP and potentially have a higher capital cost. So the case is not so straightforward to my mind.

[Jeremy Harris]

I started another thread on smart meters, to avoid deflecting this one.

I found that even adding lots of additional insulation around an already double-foam-sprayed thermal store (i.e. one with near double the normal insulation level to start with) still didn't reduce the losses a great deal (IIRC it dropped from around 3.5 kWh/day to around 2.5 kWh/day at 65 deg C, for a 260 litre TS with a rated standing heat loss of about 1.5 kWh/day at 75 deg C, using the test spec they use).  Against this, the Sunamp PV loses around 600 Wh/day and runs 10 deg hotter to start with than the TS (I ended up turning it down to 65 deg C, because at 75 deg C the TS losses were even higher and seriously overheating the services room).

The difference between the Sunamp and a thermal store isn't the insulation at all, the effect of the insulation on the Sunamp performance is minor, the major standing loss saving comes from using a phase change material, that doesn't sit at a high temperature when it is charged, but only releases heat on demand, via selective phase change.

If I was to guess, then I'd say our old 260 litre thermal store, with double-sprayed insulation plus an added foamed-on layer of 50mm PIR foam, would have had a standing heat loss of around 3 kWh/day at 75 deg C, the original design temperature.

 

[SteamyTea]

3 hours ago, JSHarris said:

The difference between the Sunamp and a thermal store isn't the insulation at all, the effect of the insulation on the Sunamp performance is minor, the major standing loss saving comes from using a phase change material, that doesn't sit at a high temperature when it is charged, but only releases heat on demand, via selective phase change.

I am not totally sold on that explanation.  It is true that the usual temperature of the Sunamp is lower than the design temp of a normal thermal store.  But if you store an equivalent amount of energy, at the same temperature in a thermal store, which has a smaller surface area, even though it has a higher U-Value, I would think losses are comparable if the Sunamp did not use such good insulation.

Would have to sit down and work it out, but got a show on for the next few weeks, so going to be a bit busy.

Edited July 8, 2016 by SteamyTea

[Jeremy Harris]

I can say that when charged the Sunamp PV is cool, not hot.  I'm not surprised by this, as that is exacltly how sodium acetate behaves in things like hand warming pads.  When it's a liquid (i.e. "charged" to it's maximum heat capacity) then it stays at the temperature of its environment.  When the phase change is triggered by a nucleation device (the small "clicker" inside the hand warmers) then heat is released.

What I don't fully understand is how nucleation is controlled, but that, together with the precise make up of the form of sodium acetate they use is, I think, one of the keys to the way it works.

I remember discussing this years ago on the GBF, and concluding that the key to making a PCM store work was finding a way to control nucleation and the subsequent phase change it triggers.  If I was to guess, then I'd say there if the heat release is controlled, such that the storage cell can reach a high temperature when a nucleation event has started, but heat has stopped being taken from the storage cell, then there may well be a stable situation in which part of the storage cell is in the high energy, liquid, form and part is in the low energy solid form, and that some how nucleation stops and a state of equilibrium is reached, where the storage cell temperature gradually reduces, with most of the stored heat being in the remaining liquid phase of the sodium acetate.

I've had the lid off mine a couple of times, and removed the top insulation, and can say the cells are not very hot when fully charged, which I am sure is a key part of the low overall standing losses.  The insulation definitely isn't fantastic, it looks like 20 to 25mm thick vacuum insulation panels, which is about the same as 60 to 75mm of PIR foam (lambda for a VIP is around 0.007 I think, that for PIR is around 0.022).

[Stones]

7 hours ago, SteamyTea said:

Tricky one.

Cylinder losses are uncontrolled loses, so not that useful.  And they are probably larger than you think.

Really comes down to the basic arithmetic in the end:

Reasonable Expectation of Usage + Acceptable Losses = Real Usage

Price of Different Technologies / Longevity + Running Price = Real Unit Price

In our last house we kept our cylinder at 50C.  Cylinder and pipe losses (similar to pipe runs I initially proposed) were 3.5kWh per day, so that is what I have been basing my calculations on.  For 6 months of the year, possibly more, those losses would be useful, but otherwise they have the potential to add to overheating, although given the very interesting discussions elsewhere about the impact of solar gain on houses built by the same company and to the same standard, I think that has the greater potential to be problematic rather than cylinder losses per se.  I am working through the options using the method you suggest, but the additional variables over which I have no control, do complicate things.  

6 hours ago, Alphonsox said:

I'm still of the opinion that the Sunamp ticks most of the boxes. Although potentially a more expensive solution the control of losses and small size make sense to me. In our case the main decision is whether it makes sense to install PV or to use E7. If I had access to E10 electricity the decision would be made.

It does indeed tick a lot of boxes, and probably makes more sense as house occupancy reduces (when losses become a bigger share of DHW energy requirement).  I think the biggest issue is the limited availability of E10 tarrifs, how long they will be available for and how pricing will change with smart metering.  I need to be able to charge my DHW store prior to morning showers and again in late afternoon ready for the evening.  

3 hours ago, ragg987 said:

I also specified a 300l cylinder based system, then looked at Sunamp quite late in the day, but decided (with input from Sunamp) not to go with the Sunamp. Also, the cost / benefit is marginal. The 4-cell Sunamp costs £1,000 more than a 300l cylinder. If the difference in standing losses between Sunamp and cylinder is 1kWh per day, would take over 20 years to break even (simple straight line calculation without the cost of the capital included - would be longer than 20 years if included).

If you have a separate cupboard for the cylinder, why not lag it all round so that heat loss is reduced? After all, the primary difference between the Sunamp and a cylinder (from heat loss perspective) is quality of insulation. I intend giving this a go.

One other factor, if you intend to use a ASHP to fulfill your DHW needs, you will need a high temperature unit to active the PCM in the Sunamp (I think Andrew said >60 deg C). This is going to hit your COP and potentially have a higher capital cost. So the case is not so straightforward to my mind.

The choices before me: SunampPV charged directly with off peak electric (hopefully excess PV in the future), Direct UVC charged with off peak electric or Indirect UVC charged using ASHP.  A Direct UVC represents the lowest capital cost option, but highest running cost.  next up is the Indirect UVC and ASHP.  The interesting thing with that option is that upgrading the size of the ASHP and cylinder from Direct to Indirect, adds another £900 onto the cost of a Direct cylinder, significantly reducing the gap between an ASHP set up and the SunampPV.  Even so, it still means quite a number of years before the savings repay the additional capital cost.

One of the reasons I was looking at going with the Sunamp was that I could locate it in a bathroom cupboard and reduce pipe runs from the original cylinder location (20+ metres) to as little as 2 metres.  Looking at my plans again, I'm wondering whether locating the cylinder next to my UFH manifold in the hallway cupboard might be the best solution.  Puts the cylinder in the centre of the house, pipe runs reduced to 8 - 10 metres.

Decisions, decisions... 

 

[DeeJunFan]

7 hours ago, Alphonsox said:

I'm still of the opinion that the Sunamp ticks most of the boxes. Although potentially a more expensive solution the control of losses and small size make sense to me. In our case the main decision is whether it makes sense to install PV or to use E7. If I had access to E10 electricity the decision would be made.

True Dat, someone should lobby NIE