Archive for April, 2022

First Winter with a Heat Pump

April 27, 2022

Friends, our first winter with a heat pump is over.

Last week:

  • I switched off the space heating, and…
  • I changed the heating cycle for domestic hot water (DHW) from night-time (using cheap-rate electricity) to day-time (using free solar electricity).

From now until the end of July, I am hopeful that we will be substantially off-grid.

Let me explain…

No Space Heating 

The figure below shows the temperatures relevant to our heating system for the week commencing Saturday 9th April.

The week started cold, with overnight temperatures close to 0 °C and daytime temperatures peaking at 12 °C.

But the week ended with much warmer temperatures, and even in the absence of any heating flow, the household temperature rose above 21 °C. At this point I decided to switch off the space heating. You can see this on the monitoring data below.

Up to the 15th April, the heat pump would operate each evening – you can see this because radiator temperatures oscillated overnight as the heating circuit struggled to deliver a very low heating power.

From the 16th April – with the space-heating off – you can see the radiator temperatures simply fell after the DHW water heating cycle.

Click image for a larger version. Graph showing four temperatures during the week beginning 9th April 2022. The upper graph shows the temperature of radiator flow and the domestic hot water (DHW). The lower graph shows the internal and external temperatures. In the colder weather at the start of the week, the radiator flow temperatures cycled on and off. In the warmer temperatures at the end of the week, heating stopped automatically. On 16th April I switched the space heating circuit off.

Heating DHW during the day 

The next graph shows the same data for the following week. Now there is no space-heating in the house, but the insulation is good enough that household temperature does not fall very much overnight.

On the 20th April I switched from heating the domestic hot water at night (using cheap rate electricity) to heating during the afternoon (using electricity generated using solar PV).

My plan was that by 2:00 p.m., the battery would be substantially re-charged, and heating the hot water at that time would:

  1. Minimise exports to the grid and maximise self-use of solar-generated electricity.
  2. Heat the domestic hot water using air that was ~ 10 °C hotter than it would be at night – improving the efficiency of the heat pump.

Click image for a larger version. Graph showing four temperatures during the week beginning 16th April 2022. The upper graph shows the temperature of radiator flow and the domestic hot water (DHW). The lower graph shows the internal and external temperatures. The radiator flow was switched off. On 20th April I switched from heating the domestic hot water at night to heating during the day.

One can see that household temperature has fallen a little during the week, but only to around 19 °C, which feels quite ‘spring-like’ in the sunshine.

The big picture 

The graph below shows:

  1. The amount of electricity used by the household
  2. The amount of electricity drawn from the grid

It covers the whole of 2021 and the start of 2022 up to today (almost) the end of April. The graphs show running averages over ± 2 weeks.

Click image for a larger version. Graph showing the amount of electricity used by the household each day (kWh/day) and the amount of electricity drawn from the grid each day (kWh/day). Over the 8 months of the winter heating season, 27% was supplied by solar generated electricity.

The 4 kWp solar PV system was installed in November 2020 and was just beginning to make a noticeable difference to our electricity consumption in the spring of 2021.

In March 2021 we installed the Powerwall and immediately dropped off the grid for just over 2 months! In mid-summer we had a run of very poor solar days and we began to draw from the grid again.

In July 2021 we installed a heat pump and this extra load (for DHW) coupled with the decline in solar generation caused us to need to draw a few kWh from the grid each day.

Over the 8 month heating season from the start of August to the end of April, the household used 4,226 kWh of electricity for all the normal activities (~ 2,200 kWh) plus heating using the heat pump (~2,000 kWh). Over this period the heat pump delivered just over 7,000 kWh of heat for a seasonally averaged COP of around 3.5.

However, even in this winter season, only 3,067 kWh were drawn from the grid – mostly at low cost. The balance (27%) was solar generated.

Summer and Winter Settings

The optimal strategy for the Powerwall is now becoming clear.

In the Winter season, daily consumption can reach 25 kWh/day and solar generation is only ~ 2 kWh day. So in this season:

  • We operate the household from the grid during the off-peak hours.
  • We time heavy loads (dishwashing, tumble drying and DHW heating) to take place in the off peak hours.
  • We buy electricity from the grid to fill the battery (13.5 kWh) with cheap rate electricity – and then run the household from the battery for as long as possible. Typically we would need to draw full price electricity from the grid only late in the day.

Click image for a larger version. Images showing the time of day that we have drawn power from the grid (kW) in half-hour periods through the day. Each image shows the average for one month. The graph was assembled using data from the fabulous Powershaper software (link).

In the ‘summer’ season, daily household consumption is ~11 kWh and average solar generation is typically 15 kWh/day. So given that the battery has 13.5 kWh of storage, we can still stay ‘off-grid’ even during a periods of two or three dull days.

So during this period

  • We switch the battery from ‘time-based’ mode to ‘self-powered’ mode.
  • We time heavy loads (dishwashing, tumble drying and DHW heating) to take place in the afternoon.

This year and last year 

Last year (2021), as soon as we installed the Tesla Powerwall battery, we dropped off-grid within days.

But this year (2022) we have an additional daily electrical load. Now we are heating DHW electrically with a heat pump which requires ~ 1.5 kWh/day.

Nonetheless, I hope it will be possible to remain substantially ‘off-grid’ for the next few months. Time will tell.

I’ve been interviewed for a Podcast!

April 25, 2022

I’m on a Podcast! (Link)

Friends, I have been relatively quiet lately because I have been indulging some of my other retirement enthusiasms.

Some of these are a source of delight – such as singing and recording songs – and others are not so delightful – such as trying to clear out the detritus of my life. Throwing things out is hard.

I have been writing blog articles, but somehow the simple topics I have chosen to write about have become more complicated as I have written the articles. This is another of example of the fact that I am not writing to record what I think: I am writing to discover what I think!

But my previous articles about heat pump sizing (1, 2, 3, 4) and the summary ‘Rule of Thumb’ video have been catching people’s attention! As I write 394 people have watched the summary video – more than all the other videos on the channel put together!

Part of that popularity is because Nathan Gambling, the mastermind behind the BetaTeach initiative, asked me if I would take part in a podcast.

Nathan is from a family of heating engineers and runs BetaTeach to encourage heating engineers to adopt a ‘systems approach’ that enables them to install heat pumps in all kinds of homes.

His approach contrasts with many training programs which tend to focus on how to install the products of a particular company.

I was honoured to be asked to be his guest on his BetaTalk podcast and you can hear the results of our Zoom chat here:

It is 64 minutes of truly random chat, with a vague theme linking to temperature and heat. At times I lost my way in the interview, but hopefully you won’t.

If the above link doesn’t work, then search Google for “Betatalk – the Renewable Energy and Low Carbon Heating Podcast

Could you heat your house with a hairdryer?

April 12, 2022

Click the image for a larger version. The graph shows the average electrical power (in kW) used by our heat pump to keep the 164 square metres of Podesta Towers at approximately 20.5 °C throughout the winter.  Also shown is the typical power used by a hairdryer on typical high, medium and low powers.

Friends, a chance remark on the internet intrigued me.

Someone commented that their heat pump was heating their house using less power than a hairdryer. Could that really be true?

Looking it up (link), I found that a hair dryer actually uses rather more power than I had supposed: somewhere between 850 watts (0.85 kW) and 1850 watts (1.85 kW) depending on its power setting.

I then looked up week-by-week data for our heat pump at Podesta Towers.

And slightly to my surprise I found that even in the coldest weeks, the average electrical power used by the heat pump was less than 800 watts (0.8 kW) i.e. we were heating our house with less electricity than it takes to run a hairdryer – on its lowest setting! And that includes re-heating the hot water tank each day!

So why didn’t I just buy a hairdryer?

Why? Because a hairdryer – even on full power – could not heat my house.

The wonder and fascination of heat pumps is that they don’t just squander the electricity they consume: they use it to scavenge heat from the outside air and even on the coldest days they can deliver many times more heat energy into the house than the electrical energy they consume.

The ratio of the heat energy they deliver to the electrical energy they consume is called the Coefficient of Performance (COP) and for my heat pump the average COP since installation is 3.6.

In other words the heat pump has delivered more energy than two hairdryers on full power while consuming less energy a single hair dryer on low power.

COP

The graph below shows the COP evaluated week-by-week and the average value since August 2021.

Click the image for a larger version. The graph shows the average coefficient of performance (COP) week-by-week since installation in August 2021. Also shown in the average coefficient of performance (COP) since installation, also known as the Seasonal Coefficient of Performance (SCOP).

The low average values in the autumn are because the heat pump is only delivering domestic hot water at 55 °C and is not heating the house at all.

During this time, the 20 watts of electrical power that the heat pump’s computer consumes (0.5 kWh/day) represents a significant fraction of the energy delivered.

In contrast, in winter the heat pump is delivering more than 20 kWh of heat energy per day and the consumption by the heat pumps’ control circuitry is less than 3% of the heat energy delivered.

Summary

I found this juxtaposition intriguing. 

A hair dryer is a simple device – a hand-held fan heater – and a heat pump is a much more complex machine.

But comparing them just by their electrical consumption highlights the awesome power of heat pumps.

March 2022

April 8, 2022

Friends, Spring is springing, and our first winter with a heat pump is ending.

Overall, it has been phenomenally successful. All the parts of our refurbishment have played their part.

  • The triple-glazing and external wall insulation have reduced the heating power required to heat the house.
  • The solar panels continued to deliver ~5% of our electricity requirements even in December.
  • The battery (a 13.5 kWh Tesla Powerwall) allowed us to download cheap electricity at night and use it to heat the house during the day.
  • The heat pump kept the house warm and delivered hot water, with an average Coefficient of Performance of around 3.5.

In this article I will be looking at figures for the month of March 2022.

When the heating season is a little more over than it is at present, I will write about the winter as a whole.

Solar PV and Battery

Click image for a larger version. This is on the notice board outside my house.

I have put a notice on the front of the house to advertise how little we are spending on heating and running the house. Excluding the standing charges, we spent just £14.34 heating the house and running all the electrical items in the house.

In honesty, I am embarrassed to disclose how little I am spending on fuel bills. I am embarrassed because of the suffering and anxiety that so many people will be feeling now as prices rise.

Nonetheless, when it comes to communicating the wonder of a well-insulated home powered by solar, talking about money is one way to communicate more viscerally than using kilowatt-hours and kilograms of carbon dioxide.

Energy Flows

Click image for a larger version. This graphic shows my best estimate of the energy flows around my house. There are two sources of electricity: the grid and the solar PV system. During the hours when grid electricity is cheap, the grid supplies the house directly and charges the battery. Solar PV supplies the house directly, then if household demand is met, it charges the battery, and if the battery is full, it exports electricity. My analysis suggests that the battery is only 87% efficient i.e. 13% of the energy is lost in the process of charging and discharging the battery.

The graphic above describes the energy flows in the house.

On a typical day:

  • Between 00:30 and 04:30 the house runs on cheap grid electricity, and we time the dishwasher and hot water heating to run over this period. The grid also charges the battery.
  • After 04:30 the battery runs the household and is then re-charged during daylight hours by whatever solar PV is available.
    • If the battery charge reaches 100%, then solar PV is exported.
    • If the battery discharges to 0%, then we run off full price grid electricity.

Analysing the data from the Tesla App, it looks like the battery returns 87% of the charge delivered to it. The system is specified to have a charge/discharge efficiency of 90%. I suspect that extra losses arise from the energy the battery uses to maintain its own condition.

The figure below shows the average pattern of grid use during the month. The majority of electricity is used during the cheap rate period and only a small fraction of full-price electricity is required on days when solar PV generation is insufficient to keep the battery topped up.

Click image for a larger version. This graphic shows the time of day at which the house drew electricity from the grid in March 2022. The vast majority of the electricity was consumed at night to (a) charge the battery and (b) directly operate timed loads such as the dishwasher, washing machine, and heat pump domestic hot water cycle.

Heat Pump

Click image for a larger version. Graph showing internal and external temperatures, and the temperature of water flowing in the radiators during the month of March. Data were collected every 2 minutes. The radiator flow temperature data has been smoothed. It is clear the system operates well to keep the internal temperature constant even as the external temperature varies

The average external temperature was 9.3 °C, but the month started very cold, and then later there were some exceptionally warm days (with cold nights).

The weather compensation adjusted the flow temperature in the radiators to keep the internal temperature at a comfortable average of 21.1 °C

The monthly averaged Coefficient of Performance was 3.75 which is rather more than I had hoped for.

Conclusion

When we installed the battery in March 2021, we immediately dropped of the grid for 90 days: this felt astonishing. But back then then our heating was with gas.

Now our heating and hot water systems are electrical and this adds to the daily load.

As the year progresses, Solar PV generation is growing and heating demand is falling. At some point I hope we will again be able to reduce grid use to zero for an extended period – but it will definitely not be as long as last year.

It was interesting to arrive at a figure for the battery storage efficiency. The figure of 87% was lower than I had hoped for, but since the battery is saving us so much money, it seems churlish to complain!

 

 

What Size Heat Pump Do I Need? A Rule of Thumb

April 5, 2022

Friends, a few weeks ago I wrote four articles about using the idea of Heating Degree Days to make simple calculations about heat losses from one’s home.

  • Article 1 was an introduction to the idea of Heating Degree Days as a general measure of the heating demand from a dwelling.
  • Article 2 explained how and why the idea of Heating Degree Days works.
  • Article 3 looked at the variability of Heating Degree Days across the UK, at locations around London, and from year to year.
  • Article 4 introduced some rules of thumb for estimating the Heat Transfer Coefficient for a dwelling and the size of heat pump it requires.

The Rule of Thumb for Heat Pump Sizing is dramatically simple:

The video above is about using these ‘Rules of Thumb’.

I feel these rules could be helpful to both heat pump installers and their clients.

The Powerpoint slides (.pptx) I use in the presentation can be found here.

 

 

 

 

 


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