Benefits of a thermal store?

[Declan52]

7 minutes ago, ProDave said:

Well my 5KW HP is doing a fine job of keeping the house warm, running at a very low duty cycle. I cannot imagine anyone needing a 17Kw version.

 

In my ideal world, I would not even try heating the DHW until mid day.  I am a "shower at night" person, I just can't see the logic in getting into bed dirty and waiting until the morning to get clean, so I just need a bit of warm water in the morning for a face wash.  But the girls seem to want a piping hot shower at the crack of dawn which forces a DHW top up early in the morning, when the day has not had chance to warm up, and (thinking of the future) not even any sun for the PV

Maybe that's the way to go.  Just run a smaller hp for longer.  I could  give the tank a 2 hr bump in the evening and then another 2hrs in the morning. 

We are all shower in the evening types.  Think I would drown if I took a shower in the morning as I am not  exactly  alert first thing. 

[dpmiller]

What about the "high temp" units http://www.earthsaveproducts.com/product/high-temp-12kw-air-source-heat-pump/

I believe some are just r134a filled, but there are others that are dual compressor/ stage?

[Stones]

Worth remembering that thermostatic showers need a temperature differential between incoming DHW temp and your selected output temp to work/mix properly. For most I believe that differential is around 6 - 10C.

 

I costed out using an ASHP to satisfy all DHW provision, and the preheat to 40C then top up with immersion alternative.

 

Heating to 50C and based on  300litres, and an ASHP COP of 2.4, I found the cost to be the same. Sure the ASHP electricity cost for preheat was less due to higher COP, but this was offset by the immersion top up cost.

 

If it costs the same, is there really any benefit to complicating your DHW system / operation? I concluded there wasn't.

 

I should point out that I don't have any PV (diversion) to DHW.

 

 

 

[Jeremy Harris]

1 hour ago, dpmiller said:

What about the "high temp" units http://www.earthsaveproducts.com/product/high-temp-12kw-air-source-heat-pump/

I believe some are just r134a filled, but there are others that are dual compressor/ stage?

 

I'd look carefully at the full spec, as they quote a "headline" COP of 4.23, but then go on to say that the COP is 3.55 when tested at 1 deg C to 2 degC, which is about average.  They also don't mention the humidity for the test undertaken just above freezing, or the flow temperature.  The unit will be imported from China for sure, as ESP are importers of Chinese domestic products in the main, not that this is an issue as long as support is available.  They don't specifically state that it's a two-stage heat pump, just that it has two compressors, and if I was to guess I'd say that it's not two-stage.  I'd also take a punt and say that it's not inverter controlled, either, and that the dual compressor design is a way to stage the capacity but still use cheaper direct drive compressors.

 

There are two stage series units around, where the uplift is staged to maintain a high COP for each heat pump, with the first stage being an air to refrigerant (in a refrigerant to refrigerant heat exchanger) heat pump and the second stage being a refrigerant to water heat pump.  Although these can give a higher flow temperature, the increased losses mean that there isn't a significant advantage in terms of overall COP.  I believe this may be one reason why they don't seem to be that readily available.

 

One thing worth looking at is whether the COP is really that significant for DHW.  When trading off initial capital cost versus running cost, and taking account of the number of really cold days we have in a year, accepting that a heat pump COP may drop to around 2 to 2.5 when delivering hot water on a few dozen days a year may not be a big deal.  The hard part is getting hold of representative performance data for any heat pump, in order to work this out.  The standard tests are pretty useless (a bit like standard car fuel consumption tests) and what's needed is the performance over a wide range of operating conditions, which isn't readily available for a lot of the budget units, or even some of the well-known brands.

[PeterW]

4 minutes ago, JSHarris said:

The standard tests are pretty useless (a bit like standard car fuel consumption tests) and what's needed is the performance over a wide range of operating conditions, which isn't readily available for a lot of the budget units, or even some of the well-known brands.

 

That’s what made the BSI Report for those IVT units quite interesting reading @JSHarris as they did a wide range of tests and you get a real world example - or as far as you can in a lab. 

 

IVT / Mitsubishi V90

 

They were getting sub 2 scores on CoP in tests 19 & 20 which are the near freezing and high output tests, but that is still double the CoP of direct electric. 

 

What is also nice is they explain the full calculation for CoP at the beginning ..!

[Ricco]

On 29/11/2018 at 17:48, JSHarris said:

 

There are two different units of measurement here, power and energy, and to compare anything they need to be separated out. 

 

Taking a Sunamp UniQ 9, or a 210 litre hot water cylinder (they have pretty much the same heat energy storage capacity) as examples, then if both had a 3 kW nominal heating element both would take about 3 hours to be fully charged with around 9 kWh of heat energy (heat energy, in kWh, is heat power in kW x time in hours).  In practice both will take a bit over 3 hours to be fully charged, due to losses and slight variations between the two, but we can assume for this example that they both hold around 9 kWh of heat energy when charged.

 

In terms of heat losses, then a typical well-insulated hot water cylinder will lose between 2 kWh and 2.5 kWh over 24 hours, whereas a Sunamp UniQ 9 would lose around 0.7 kWh, so the losses are a lot lower for the Sunamp.

 

If heating either a hot water Sunamp or a hot water tank from a heat pump, then the heat pump performance will not be good, because of the high temperature required.  I'm not even sure that many heat pumps would deliver a high enough flow temperature to charge a hot water Sunamp, as they need around 60 deg C or more.  You can get away with having a hot water system that runs at around 50 deg C, but when using a hot water cylinder you'd be advised to fit a larger capacity one, because it will be running at a lower temperature, so won't get mixed down with cold water at the point of use by as big a ratio, so more water from the tank will be drawn off than might be the case if the tank was running at a higher temperature.

 

How big a hot water tank you need really depends on your hot water needs.  For example, we run two showers per day, plus washing up, hand washing etc, and use around 150 litres of water at around 58 deg C (from an electrically heated Sunamp UniQ) per day, sometimes a bit less, sometimes a bit more.  I'm in the process of measuring the energy (in kWh) that we use to generate hot water, and should have a reasonable figure as a daily average by early next week.  Right now it looks as if we are using around 4 kWh or so of electricity per day to heat our hot water (roughly 20% from solar panels, 80% from the grid, as it's winter), but that ignores the ASHP pre-heat system we have, which feeds warm (around 35 deg C to 40 deg C) water to the Sunamp, so reducing our electricity usage a fair bit.

 

So, to work out the relative costs, both in terms of the initial investment plus the running cost, we need to know how much hot water you are likely to use.  Heating is a different issue, and in a well-insulated and airtight house will often be lower than the hot water requirement, and is probably best dealt with separately.

 

Just to go back to what was said above - if the example @JSHarris used with a UniQ 9 or a 210L cylinder and both were charged using an immersion then they would both cost approximately the same to “charge” fully. But if the cylinder was “charged” using an ASHP then would the benefit of the c o p (even if it was poor) still be more cost effective than using an immersion. (For this example take that all electricity is being bought from the grid).

 

So is the big benefit of a SunAmp the fact that is has such a low standing loss?

 

Still just trying to get my head round the best DHW solution so apologies for the continued questions! 

[Jeremy Harris]

Yes, the primary benefit of the Sunamp is the much lower heat loss, together with the much smaller space it takes up.

 

Charging with a heat pump presents a slightly different challenge, in that whilst water can just be heated up to any temperature and used to store heat, the Sunamp uses a phase change material (PCM), so needs to be heated to a temperature above the phase transition point in order to store the rated amount of heat energy.  The transition temperature for the hot water Sunamp units is 58 deg C, but a heat pump would need to exceed this by a few degrees to ensure that all the PCM had changed to the liquid phase.

 

This means that a heat pump with a relatively high flow temperature is needed, in excess of 60 deg C, and that creates an issue with COP, especially in cold and wet weather, where icing may be a problem.

[Ricco]

But even with a poor COP from the heat pump would it be more economical than using an immersion heater to charge the SunAmp? (Again buying the electricity straight off the grid)

[Jeremy Harris]

24 minutes ago, Ricco said:

But even with a poor COP from the heat pump would it be more economical than using an immersion heater to charge the SunAmp? (Again buying the electricity straight off the grid)

 

Probably.  At a guess the COP might drop to as low as 2 on occasions, but that's still cheaper than using peak rate electricity directly to an immersion heater.  The key thing would be getting a heat pump that could reliably provide a flow temperature of over 60 deg C under all conditions, but that should be possible.  @PeterW has posted some interesting performance data on the IVT units that indicates that they can probably manage this OK.

[Gone West]

14 hours ago, Ricco said:

But even with a poor COP from the heat pump would it be more economical than using an immersion heater to charge the SunAmp? (Again buying the electricity straight off the grid)

We use an EASHP to heat our DHW tank and heat it to only 45 degrees. That temperature seems to work well for us and of course there are a lot less standing heat losses from the tank when the storage temperature is a lot lower. I don't know what the COP is but it does seem very efficient judging by our electricity bill. We don't have any PV as we are still trying to justify spending money on it bearing in mind our electricity usage is so low. We would install PV if we had an EV.

[Ricco]

I would be interested in the results of your energy usage when you have them @JSHarris

 

I am thinking of a similar setup @PeterStarck I cannot justify the capital outlay of PV at the moment.

Did you oversize the UVC as you are only heating it to 45 degrees?

 

Our SAP calculations shows our house will need the following... 

 

Water Heating Energy Requirements (kWh/year): 1,786

Space heating requirement (kWh/year): 12,984.35

 

I believe the water heating requirements are massively under estimated as at the moment we use 2 showers and some hand washing per day and a few baths each week, but with two young kids the hot water demand will only grow so needs to be apexes for the future - I’m guessing maybe 200+ litres per day. 

[Jeremy Harris]

17 minutes ago, Ricco said:

I would be interested in the results of your energy usage when you have them @JSHarris

 

 

 

I've just posted the results from the first week of monitoring in this thread (click the arrow at the top right corner below to go directly to the post):

 

[Declan52]

Being able to  use any excess pv I generate to heat the water up no matter what!!! ? 

[Gone West]

19 hours ago, Ricco said:

 

I am thinking of a similar setup @PeterStarck I cannot justify the capital outlay of PV at the moment.

Did you oversize the UVC as you are only heating it to 45 degrees?

 

Our UVC is part of the Genvex Combi and is 185l. The water is heated by an EASHP which is built into the Genvex Combi along with the ventilation unit. There are only two of us in the house and we use water saving measures as much as we can. We have never yet run out of hot water. It doesn't sound as though 185 litres would be enough for your usage.