Posts Tagged ‘Carbon dioxide emissions’

What it takes to heat my house: 280 watts per degree Celsius above ambient

August 16, 2019

The climate emergency calls on us to “Think globally and act locally“. So moving on from distressing news about the Climate, I have been looking to reduce energy losses – and hence carbon dioxide emissions – from my home.

One of the problems with doing this is that one is often working ‘blind’ – one makes choices – often expensive choices – but afterwards it can be hard to know precisely what difference that choice has made.

So the first step is to find out the thermal performance of the house as it is now. This is as tedious as it sounds – but the result is really insightful and will help me make rational decisions about how to improve the house.

Using the result from the end of the article I found out that to keep my house comfortable in the winter, for each degree Celsius that the average temperature falls below 20 °C, I currently need to use around 280 W of heating. So when the temperature is 5 °C outside, I need to use 280 × (20 – 5) = 4200 watts of heating.

Is this a lot? Well that depends on the size of my house. By measuring the wall area and window area of the house, this figure allows me to work out the thermal performance of the walls and windows. And then I can estimate how much I could reasonably hope to improve the performance by using extra insulation or replacing windows. These details will be the topic of my next article.

In the rest of this article I describe how I made the estimate for my home which uses gas for heating, hot water, and cooking. My hope is it will help you make similar estimates for your own home.

Overall Thermal Performance

The first step to assessing the thermal performance of the house was to read the gas meter – weekly: I did say it was tedious. I began doing that last November.

One needs to do this in the winter and the summer. Gas consumption in winter is dominated by heating, and the summer reading reveals the background rate of consumption for the other uses.

My meter reads gas consumption in units of ‘hundreds of cubic feet’. This archaic unit can be converted to energy units – kilowatt-hours using the formula below.

Energy used in kilowatt-hours = Gas Consumption in 100’s of cubic feet × 31.4

So if you consume 3 gas units per day i.e. 300 cubic feet of gas, then that corresponds to 3 × 31.4 = 94.2 kilowatt hours of energy per day, and an average power of 94.2 / 24 = 3 925 watts.

The second step is to measure the average external temperature each week. This sounds hard but is surprisingly easy thanks to Weather Underground.

Look up their ‘Wundermap‘ for your location – you can search by UK postcode. They have data from thousands of weather stations available.

To get historical data I clicked on a nearby the weather station (it was actually the one in my garden [ITEDDING4] but any of the neighbouring ones would have done just as well.)  I then selected ‘weekly’ mode and noted down the average weekly temperature for each week in the period from November 2018 to the August 2019.

Weather history for my weather station. Any nearby station would have done just as well. Select ‘Weekly Mode’ and then just look at the ‘Average temperature’. You can navigate to any week using the ‘Next’ and ‘Previous’ buttons, or by selecting a date from the drop down menus

Once I had the average weekly temperature, I then worked out the difference between the internal temperature in the house – around 20 °C and the external temperature.

I expected that the gas consumption to be correlated with the difference from 20 °C, but I was surprised by how close the correlation was.

Averaging the winter data in the above graph I estimate that it takes approximately 280 watts to keep my house at 20 °C for each 1 °C that the temperature falls below 20 °C.

Discussion

I have ignored many complications in arriving at this estimate.

• I ignored the variability in the energy content of gas
• I ignored the fact that less than 100% of the energy of the gas is use in heating

But nonetheless, I think it fairly represents the thermal performance of my house with an uncertainty of around 10%.

In the next article I will show how I used this figure to estimate the thermal performance – the so-called ‘U-values’ – of the walls and windows.

Why this matters

As I end, please let me explain why this arcane and tedious stuff matters.

Assuming that the emissions of CO2 were around 0.2 kg of CO2 per kWh of thermal energy, my meter readings enable me to calculate the carbon dioxide emissions from heating my house last winter.

The graph below shows the cumulative CO2 emissions…

Through the winter I emitted 17 kg of CO2 every day – amounting to around 2.5 tonnes of CO2 emissions in total.

2.5 tonnes????!!!!

This is around a factor of 10 more than the waste we dispose of or recycle. I am barely conscious that 2.5 tonnes of ANYTHING have passed through my house!

I am stunned and appalled by this figure.

Without stealing the thunder from the next article, I think I can see a way to reduce this by a factor of three at least – and maybe even six.

Christmas Bubbles

December 23, 2018

A time-lapse photograph of a glass of fizzy wine.

Recently I encountered the fantastic:

Effervescence in champagne and sparkling wines:
From grape harvest to bubble rise

This is a 115-page review article by Gérard Liger-Belair about bubbles in Champagne, my most favourite type of carbon dioxide emission.

Since the bubbles in champagne arguably add £10 to the price of a bottle of wine, I guess it is worth understanding exactly how that value is added.

I found GLB’s paper fascinating with a delightful attention to detail. From amongst the arcane studies in the paper, here are three things I learned.

Thing 1: Amount of Gas

Champagne (and Prosecco and Cava) have about 9 grams of carbon dioxide in each 750 ml bottle [1].

Since the molar mass of carbon dioxide is 44 g, each bottle contains approximately 9/44 ~ 0.2 moles of carbon dioxide.

If released as gas at atmospheric pressure and 10 °C, it would have a volume of approximately 4.75 litres – more than six times the volume of the bottle!

This large volume of gas is said to be “dissolved” in the wine. The molecules can only leave when, by chance, they encounter the free surface of the wine.

Because the free-surface area of wine in a wine glass is usually larger than the combined surface area of bubbles, about 80% of the de-gassing happens through the liquid surface [2].

Thing 2: Bubble Size and Speed

But fizzy wine is call “fizzy” because of the bubbles that seem to ceaselessly form on the inner surface of the glass.

Sadly, in a perfectly clean glass, such as one which has repeatedly been through a dishwasher, very few bubbles will form [3].

But if there are tiny cracks in the glass, or small specks of dust from, for example, a drying cloth, then these can trap tiny air bubbles and provide free-surfaces at which carbon dioxide can leave the liquid.

At first a bubble is just tens of nanometres in size, but it grows at a rate which depends upon the rate at which carbon dioxide enters the bubble.

As the bubble grows, its surface area increases allowing the rate at which carbon dioxide enters the bubble to increase.

Eventually the buoyancy of the bubble causes it to detach from its so-called ‘nucleation site’ (birthplace) and rise through the liquid.  This typically happens when bubbles are between 0.01 and 0.1 mm in diameter.

To such tiny bubbles, the wine is highly viscous, and at first the bubbles rise slowly. But as more carbon dioxide enters the bubble, the bubble grows [4] and its speed of rise increases. The rising speed is close to the so-called ‘Stokes’ terminal velocity. [5]

So when you look at a stream of bubbles you will see that at the bottom, the bubbles are small and close together and relatively slow-moving. As they rise through the glass, they grow, and their speed increases.

If you can bear to leave your glass undrunk for long enough, you should be able to see the rate of bubble formation slow as the carbon dioxide concentration falls.

This will be visible as an increase in the spacing of bubbles near the nucleation site of a rising ‘bubble train’.

Thing 3: Number of bubbles

Idle speculation often accompanies the consumption of fizzy wine.

And one common topic of speculation is the number of bubbles which can be formed in a gas of champagne [6]. We can now add to that speculation.

If a bubble has a typically diameter of approximately 1 mm as it reaches the surface, then each bubble will have a volume of approximately 0.5 cubic millimetres, or 0.000 5 millilitres.

So the 4.75 litres of carbon dioxide in a bottle could potentially form 4750/0.0005 = 9.5 million bubbles per bottle!

If a bottle is used for seven standard servings then there are potentially 1.3 million bubbles per glass.

In fact the number is generally smaller than this because as the concentration of carbon dioxide in the liquid falls, the rate of bubble formation falls also. And below approximately 4 grams of carbon dioxide per litre of wine, bubbles cease to form [7].

Thing 4: BONUS THING! Cork Speed

When the bottle is sealed there is a high pressure of carbon dioxide in the space above the wine. The pressure depends strongly on temperature [8], rising from approximately 5 atmospheres (500 kPa) if the bottle is opened at 10 °C to approximately 10 atmospheres (1 MPa) if the bottle is opened at 25 °C.

GLB uses high-speed photography to measure the velocity of exiting cork, and gets results which vary from around 10 metres second for a bottle at 4 °C to 14 metres per second for a bottle at 18 °C. [9]

I made my own measurements using my iPhone (see below) and the cork seems to move roughly 5 ± 2 cm in the 1/240th of a second between frames. So my estimate of the speed is about 12 ± 5 metres second, roughly in line GLB’s estimates

Why this matters

When we look at absolutely any phenomenon, there is a perspective from which that phenomenon – no matter how mundane or familiar – can appear profound and fascinating.

This paper has opened my eyes, and I will never look at a glass of Champagne again in quite the same way.

Wishing you happy experimentation over the Christmas break.

Santé!

References

[1] Page 8 Paragraph 2

[2] Page 85 Section 6.3

[3] Page 42 Section 5.2

[4] Page 78 Figure 59

[5] Page 77 Figure 58

[6] Page 84 Section 6.3 & Figure 66

[7] Page 64

[8] Page 10 Figure 3

[9] Page 24 Figure 16