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Considering a Sunamp based plan change


dnb

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There is nothing like making a plan and sticking to it, is there? ;) And it's a bit of fun for a quiet Saturday after long week of software testing, 1st fix wiring and plasterboarding.

 

I originally planned to fit a large water based thermal store but since the 4th gen Sumamps appeared and the control system "gremlins" appear to be resolved maybe it is time to reconsider the plan. Nothing in the house has got to the stage of preventing such a plan change. I know the new Sunamps have only been around for a few months, but does anyone have one (any size) that's currently in use? The choices of new Sunamp are a little odd. Why so many different options for very similar prices? e.g. what does the "ePV" do that the "e" doesn't given that you need an external PV diverter? I suspect the answer is that there is no difference in hardware other than volume of PCM but the controller has some different parameters.

 

I've developed a Matlab model of hot water use based on a Monte Carlo simulation of 100 years of random usage of hot water given some parameters e.g. occupancy and sizes of bath etc. This indicates we will on average need 18kWh of hot water energy per day, with a fairly tight standard deviation. I suspect the mean is an over estimate but it does align fairly well with the gas usage for the past few summers (i.e. when the bill is DHW only). I have used the model to find the ideal number of solar PV panels based on payback time for the roof (although I didn't forecast energy prices rising quite as quickly) so I have a 6.5kW system that should yeild 7000kWh per year. 

 

At face value the usage model indicates that the house should have 2x 9kWh Sunamps. This gives the right amount of hotness per day on average based on one "charge" per day and sufficient immersion heater wattage to use most of the solar power but two sunamps is an expensive option, and there are actually two "charge" opportunities most days - over night and during the day. So perhaps we could just have one of the 12kWh versions? 

 

I put a bit more effort into the model and made it generate annual costs for a 12kWh store and an 18kWh store (assuming low standing losses - way better than the thermal store case) and get the following charts:

 

For the 12wKh case, average running costs are £770 per year for importing electricity from the chart on the left using the random usage model and the random solar irradiance model. The chart on the right is the amount of solar PV not used for DHW so is in theory available for other house uses should they happen to line up in time. (No surprise that the numbers are similar when the PV provides 7000kWh over a year and the DHW requirement seems to be around 6500kWh.). This run of the model assumes we implement a scheme to put heat into the Sunamp via the solar PV driving an immersion heater external to it - e.g. something like subverting a Willis heater driving an UFH heating loop  I like this idea since it is very simple, especially if the Sunamp internal heaters are difficult to drive - they only appear to work with the Eddi controller that doesn't appear to work properly with my PV array since it's too big. Does anyone know details of this?

12kW_suunamp_cost.thumb.png.befe9ffc840ab92ce5c35b7b4d9f8d63.png

The next chart shows one year from the simulation illustrating key performance measures. Waste is where the system uses grid electricity when it was sunny the next day and the store wouldn't have been emptied by the days usage. Excess PV is obvious. Input shows grid electicity use. The store values show the contents of the store at 6AM and 8PM for each day. The fact that they sit at a constant level indicates that the store might be a bit small.

12kW_suunamp_yearly_energy.thumb.png.f1c34ec8148bbd7758afd3e6a7f42970.png

 

This is the 18kWh case. Yearly running costs are down by £300/year. So the second Sunamp will break even after around 6 years at current usage unless the excess PV was actually being used extremely efficiently.

18kW_suunamp_cost.thumb.png.78d6d06636c49b7bbaa707904a136f26.png

A year from the 18kWh store case. The parameters are the same as before, but now note that there is an excess of storage so the PV use can be more easily managed - waste is considerably reduced.

18kW_suunamp_yearly_energy.thumb.png.0e37b4a4e5b3edf81412e1f10c85ddea.png

 

Increasing the amount of storage beyond 18kWh results in dimishing returns as is no doubt fairly obvious. A 24kWh test shows imported electricity cost drops to £320 per year, but given the cost of a Sunamp doesn't seem particularly linear it might be a good option. That's tonight's analysis job. All in all a Sunamp solution compares well with the 500 litre thermal store - standing losses are much easier to manage and I can make considerable savings in not implementing an extremely well insulated cupboard for it. But this doesn't quite get me to the point where a Sunamp is only a small capital cost increase. More research is needed to create a viable solution - has anyone got two running in parallel? Did it present difficulties?

 

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That analysis seems to be over thinking it.  Your HW usage will be what it is.

 

The USP of a sun amp is and always has been smaller size for a given storage capacity (compared to water) and lower standing heat losses compared to a water tank.

 

If one of both of those USP's justifies a sun amp for you then it is worthy of consideration.

 

For me, what killed the idea stone dead was the PCM operates at too high a temperature to be charged from the relatively low temperature hot water from an ASHP so would have to be charged via a resistance heating element, so that's the electricity cost doubled then for me, which would kill all other advantages.

 

There was talk of a different PCM that would work at ASHP temperatures.  Did that ever happen?

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38 minutes ago, ProDave said:

 

There was talk of a different PCM that would work at ASHP temperatures.  Did that ever happen?

Yes, we have one.

 

We actually have four Sunamps. One powered from electricity via a heating element and the others from an ASHP. Two of them feed the heating and two are for hot water.

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47 minutes ago, ProDave said:

That analysis seems to be over thinking it.  Your HW usage will be what it is.

 

You haven't met the wife. Or been subject to her threat of dire consequence if the hot water should ever stop her having a bath. ?

 

49 minutes ago, ProDave said:

There was talk of a different PCM that would work at ASHP temperatures.  Did that ever happen?

They are quoting 65 deg C for charging with heat pump and 75 deg C for boilers, so guessing "not really" for that.

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2 hours ago, ProDave said:

That analysis seems to be over thinking it.  Your HW usage will be what it is.

 

The USP of a sun amp is and always has been smaller size for a given storage capacity (compared to water) and lower standing heat losses compared to a water tank.

 

If one of both of those USP's justifies a sun amp for you then it is worthy of consideration.

 

For me, what killed the idea stone dead was the PCM operates at too high a temperature to be charged from the relatively low temperature hot water from an ASHP so would have to be charged via a resistance heating element, so that's the electricity cost doubled then for me, which would kill all other advantages.

 

There was talk of a different PCM that would work at ASHP temperatures.  Did that ever happen?


Also simplified install and system configuration vs UVC, and long term inspection requirement for UVC fail safes. 
 

Not sure if these do much to offset the long term cost of resistance heating. 
 

I do like having a separate system for DHW and heating, but clearly this is less efficient. 

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