Archive for the ‘Electricity Generation’ Category

Cheap Electric Cars Are Coming!

October 3, 2021

Friends, back in March 2021 I wrote about the Wuling HongGuang – the most popular electric car in China – a car with a base model starting at $4,200.

Looking through the Fully-Charged channel’s coverage of last year’s Cheng Du motor show, many similar models are being manufactured in China – models which fill in all the gaps between the ultra-cheap and the normal price of an EV in the UK  – close to £30,000.

I’ve posted the video above, but I have also compiled a table of the cars showing a basic cost, battery size, and range.

Click for a larger version: Table showing basic statistics of cars discussed in the video above. All the numbers are uncertain and subject to change, but range of numbers is sufficient to show that EV’s can be made much cheaper than models currently on offer in the UK. Most prices are rounded up.

Summary: the future

From a Chinese perspective, I guess the UK looks like a small island in the Far West. But eventually their manufacturing capability will turn to exporting some of these models to us.

As I wrote previously, the mere existence of these models confirms that EV’s are actually simpler and cheaper to manufacture than ICE vehicles. And many of these models will be well-suited to retired people living in Teddington. And within their restricted budgets.

Of course, just replacing the overwhelming number of ICE vehicles with an equivalently overwhelming number of EV’s does not solve the UK’s problems with carbon emissions.

But the possibility that in a few years normal people will be able to buy EV’s with a range of properties and prices to match their needs does make clear that a realistic pathway is emerging for finally transitioning away from fossil-fueled vehicles.

And once that transition begins it will be hard to stop.

EV’s are cheaper to run and petrol stations run on low margins. As the amount of fuel purchased declines, petrol stations will go out of business, and in an ironic twist, ICE vehicle drivers will find themselves with “range anxiety”. They will have to carefully plan their journeys so as to be sure of finding an old-fashioned ‘filling station’.

Change is coming!

Energy Storage with Bricks – a Really Bad Idea

September 22, 2021

Friends, the intermittent nature of many renewable energy resources makes energy storage critical for any future renewable electricity network.

But the amounts of energy that need to be stored are immense.

In round numbers, to store one day of electricity for the UK requires 1 terawatt-hour of storage.

  • 1 kWh is the unit of electrical energy used by electricity companies on our domestic electricity meters – typically it costs about 20 p for us to buy and about 5 p for the companies to buy.
  • 10 kWh is roughly the amount of electricity my wife and I use in a day.
  • 1000 kWh is 1 Megawatt hour (MWh)
  • 1000 MWh is 1 Gigawatt hour (GWh)
  • 1000 GWh is 1 Terawatt hour (TWh)

And multi-day storage is just a multiple of that.

This requirement – and the business opportunities available to those who can compete in this market – have driven people to consider all kinds of off-beat ideas.

This article is about one idea which is really stupid, which will never work, but which is apparently worth over a billion dollars.

Existing Energy Storage 

The hot money in energy storage is in electrical batteries of all kinds.

Tesla (for example) can supply a collection of batteries that occupies a football field or so with the following (rounded) specifications

  • 130 MWh of storage (0.01 % of 1 TWh)
  • 100 MW charge and discharge rate
  • Efficiency ~ 90% – some energy is lost in the charge-discharge process.
  • 100 million US dollars in 2020
  • $0.8M is the cost per MWh

Over the course of time I would expect this kind of facility to get cheaper and better.

The biggest storage facility in the UK is the Dinorwig Power Station in North Wales.

  • 9.1 GWh of storage (0.9 % of 1 TWh)
  • 1.7 GW discharge rate – charges more slowly
  • Efficiency ~ 75% – energy is lost in the charge-discharge process.
  • 500 million US dollars in 1984 – about 2 billion US dollars now
  • $0.2M is the cost per MWh

Dinorwig works by pumping water between two lakes with a height different of 500 m. The mathematics is easy to do.

The stored energy (in joules) is the calculated by multiplying three numbers which school students learn as mgh

  • m is the mass of water stored in the upper lake (in kilograms)
  • g is strength of gravity – roughly 10 newtons of force for each kilogram of mass
  • h is height difference (in metres)

The discharge rate (in watts) is also calculated by multiplying three numbers

  • The mass of water per second flowing through the turbines (kilograms per second)
  • g is strength of gravity – roughly 10 newtons of force for each kilogram of mass
  • h is height difference (in metres)

Compare and contrast 

Dinorwig is massive – storing almost 1% of the UK’s daily electricity requirements and and the storage is cheap per unit of energy stored.

But there are – as far as I know – no other sites in the UK with similar potential.

Tesla batteries can be placed anywhere but they are relatively expensive.

We can foresee that there will be technological innovation, and mass-production effects that will reduce the costs and improve the performance of batteries in coming decades.

However there is nothing we can do to substantially improve the performance of Dinorwig. The simple formula mgh limits all gravity-based storage systems.

To get good performance, one needs a big mass (m) lifted up, and then dropped from, a great height (h).

No technological innovations can beat mgh.

  • But is there a gravity-based storage system which doesn’t need a unique geography?
  • Something which could be built out in modular form like Tesla’s battery farms?

Here’s the stupid idea: Project Jenga

The idea – from a company called Energy Vault –  is to store energy by building a pile of bricks.

Their videos make it seem a superficially clever idea, but I can’t get the visions of Jenga out of my head.

Basically a robot crane system uses electricity to build a tower out of very large bricks. This is equivalent to ‘charging’ a battery.

To ‘discharge’ the tower, the crane lets the bricks down onto a lower tower, and as the bricks fall they turn a generator.

Fortunately, because all gravity-storage systems are limited by the mgh equation I mentioned above, it’s possible to work out its performance parameters.

Based on information gleaned from their videos and web site I conclude that:

  • Brick size ~ 6 m x 1 m x 2.5 m – mass ~36 tonnes
  • Tower in charged state – 40 layers tall with ~ 100 bricks per layer
  • Tower in discharged state has ~ 500 bricks per layer and so is ~ 8 layers tall

Still from an Energy Vault video showing their concept for their Jenga-like tower. Notice they imagine this free-standing pile of bricks being built near wind-turbines

The total stored energy is mgh where:

  • m is the total mass of the tower, and
  • h is difference between the heights of the centres of mass in the two configurations, which must have the same basic volume and number of bricks.

Calculation of stored energy in the tower system. Each state (charged and discharged) is modelled as a hollow cylinder, with the discharged cylinder being 1 metre outside the charged cylinder. The volume is conserved between the two shapes. The stored potential energy is mgh where h is the difference in height between the centres of mass in the two configurations

So my estimate for the system they describe is:

  • 37 MWh of storage
  • 6.8 MW charge and discharge rate (assuming (optimistically) it takes 10 seconds to move 2 bricks simultaneously)
  • Efficiency ~ 85% is claimed.
  • Energy vault claim $18M, but I find it hard to believe it will cost less than 100 million US dollars: The bricks alone will cost around $5M in raw materials.
  • ~$3M is the cost per MWh

So the system costs more per MWh than a battery-based system, with no potential for future technological improvements.

Why it won’t work

For a 37 MWh Energy Vault device, charging and discharging requires building, and then dismantling, a structure the height of Canary Wharf Tower, at 240 metres tall, the UK’s third tallest building.

A 37 MWh Energy Vault store would be 240 m tall: the height of Canary Wharf Tower. Charging would involve building such a tower from free-standing bricks in about 6 hours. Mmmmm.

The 37 MWh of stored energy in such a structure, when sold as electricity at 20p/kWh, would be worth – optimistically – around £10,000. The company profits would then be the difference between the sale and the purchase price of the electricity – let’s guess £5,000 per dis-assembly/assembly cycle.

  • Nominally the system would take a few hours to build (charge) and dismantle (discharge)
  • Can you imagine building anything the size of Canary Wharf in a few hours for £5000?

The charged structure would be free-standing with no reinforced concrete or steel beams to hold it together.

But this tower is envisaged to be deployed in open country, perhaps  near wind turbines – i.e. where its often windy!

Later versions of the Energy Vault concept have a different format – with mass movements taking place using some clever un-revealed geometry inside a building which looks like it is only about 40 m tall, but spread out over a much larger area.

A still from another video showing a newer version of EnergyVault enclosed in a frame inside a building. It appears to be only – maybe – 40 metres tall.

But no matter how clever they are, they can’t escape mgh.

If the building is 40 m tall, then the centre of mass is at most 20 m off the ground.

For the same 4000 bricks they used in the ‘Jenga’ design, the uncharged area of all bricks on the ground would be 10,000 m^2 i.e. 100 m x 100 m.

If this whole 144,000 tonne structure were raised by 20 m (a likely overestimate) then the stored energy would now be just 8 MWh, storing only £1,600 worth of energy in the charged state.

But in the charged state this would now be supported by an immense (= expensive) reinforced concrete frame capable of lifting and moving these large loads.

8 MWh of storage is tiny: the equivalent of 600 Tesla PowerWall batteries (like I have in my house) which would cost around £6M but which could be bought ‘off the shelf’ with no risk.

What’s going on?

It’s not just me that has noticed that this idea is a non-starter. The video above calls out the project for its ridiculousness at great length.

But if you look at the Energy Vault website you will see story after story about investment by banks and grand plans to establish a company worth billions of pounds.

What’s going on? I have no idea: it is simply madness.

No: You are not using 100% renewable electricity

September 2, 2021

Everyone’s favourite energy company EDF recently wrote to tell me…

And by ‘changing’ they meant ‘increasing’. Roughly, the price of electricity is going up by about 10% apparently because “The wholesale cost of energy has gone up more than 50% in the last six months.”

But later in the e-mail they assured me that:

I’ll come to the asterisk in a minute, but it struck me that these two statements didn’t sit well together.

First of all, no electricity source has ‘zero’ carbon emissions: they just mean: ‘low’ carbon emissions.

Secondly, low-carbon electricity comes from either nuclear, wind, solar, biomass or hydroelectric sources, none of which have had cost increases: the increases have been mainly in the price of gas.

So one might think that EDF would be under no-obligation whatsoever to raise their prices.

Could the asterisk hold an explanation? The full asterisk text is at the end of the article but the key part is this:

All our residential tariffs are backed by 100% zero carbon nuclear electricity

Since the costs of nuclear electricity have not risen at all, one might feel further emboldened in the idea that EDF might not be obliged to raise prices after all.

However

Electricity doesn’t work like that

The way we supply electricity in the UK is complicated, and includes several ‘market’ elements. Here are two ‘explanatory sites’

At its simplest,

  • ‘Wholesale’ suppliers offer to supply electricity and ‘Retail’ companies buy from a variety of suppliers.
  • The ‘Wholesale’ companies try to sell as much electricity as they can, and make money from the difference between what it costs them to produce electricity and the ‘market price’ of electricity – which varies through the day.
  • The ‘Retail’ companies buy on behalf of their customers, and make money from the difference between what they pay and what they charge you.

The market structure is complex but aims to make sure that demand is met, and that there is a contingency against plant failures or surprise demand.

Some contracts are long-term – signed months or years in advance. And others are short-term signed only a few days before the required date of delivery. But if the price of electricity from gas-fired stations goes up – then because that electricity is essential to ‘keep the lights on’ – the market structure results in a general price increase.

But all the electricity which is supplied – from solar or nuclear or gas-fired plants or interconnectors – is ‘pooled’ to meet our collective national needs and supplied over The National Grid.

National Grid

The National Grid infrastructure is owned by a multinational for-profit company called National Grid plc. It’s major shareholders are banks.

Click for a larger version. These are the top 10 shareholders in National Grid plc as of June 2021 (source: StockZoa)

If EDF delivered their low-carbon electricity to you over EDF’s own wires, then it could potentially be low-carbon.

Similarly, if you use electricity generated on your own rooftop without calling on the National Grid at all, it can be genuinely low-carbon.

But if you have your ‘green’ electricity delivered over the grid, then it is pooled with electricity from all other sources and what you draw from the grid can – in my opinion – no longer be considered ‘low-carbon’.

Engineering not Accounting

Now you might think that if you paid for ‘green’ electricity to be ‘poured’ into the grid on your behalf, then using clever accounting you can consider the electricity you ‘withdrew’ from the grid to be ‘green’.

Unfortunately, although National Grid plc is run by accountants, the network itself operates on the principles of basic physics and engineering.

And the plain fact is, the grid doesn’t work without ALL the suppliers contributing. So in order to supply you with ‘your’ ‘green’ electricity, it is necessary to have gas-fired stations operating pretty much all the time.

And if those gas-fired power stations were switched off, the demand on the grid would exceed supply and the grid would shut down, and none of us would get ANY electricity.

So if the delivery of ‘your’ green electricity requires other people to have ‘grey’ electricity, then I don’t think that ‘your’ electricity should really be considered ‘green’.

And there’s more!

Although we can choose to use nominally ‘green electricity’ in our own home, we rely on electricity being used by lots of other people. For example:

  • In shops, and their supply chain, particularly for refrigerated products.
  • In factories that manufacture stuff we need.
  • On roads, to operate street light, traffic lights and speed cameras.
  • In hospitals.
  • In internet service centres.

And much more. This ‘other electricity’ which may be ‘grey’ or ‘green’ in terms of carbon-accounting, is in part ‘ours’ too, even when we personally are not using it.

This is the nature and power of the grid. Just like the electricity that flows in it, it is shared by us all.

So…

When I am working out how much carbon dioxide I am personally responsible for, I don’t assume that my electricity is carbon free, despite being told that it is by my electricity company!

Instead I use figures from web sites such as MyGridGB or Carbon Intensity who add up the actual sources of electricity contributed to the grid and calculate the overall carbon dioxide emissions.

  • For each kWh I draw from the grid emissions are, (in 2021) about 240 gCO2 per kWh in 2021
  • For each kWh I draw from my solar panels, after accounting for their embodied carbon dioxide, 0 grams of CO2 are emitted.

Also for each kWh of solar electricity I export back to the grid, I count the CO2 that was not emitted by a gas station because of my contribution which is about 450 gCO2 per kWh.

When I am trying to convince myself that net-zero living is achievable, I subtract these emissions from my total. But I am not sure even that is fair.

============================

The asterisk text in full

* EDF home customers get energy tariffs backed annually by zero carbon electricity as standard.

All our residential tariffs are backed by 100% zero carbon nuclear electricity, with the exception of our EV tariffs which are backed with 100% zero carbon renewable electricity.

Electricity for our GoElectric tariffs come from renewable sources such as wind, solar, biomass, tidal and hydroelectric. At the end of each fuel mix reporting year we’ll make sure we’ve purchased enough renewable electricity from EDF owned, renewable generation to match the total volume of electricity supplied to all of our customers on the GoElectric tariffs. A fuel mix reporting year begins on 1 April and ends on 31 March the following year. UK fuel mix disclosure information, published by the Government (BEIS), recognises electricity generated from wind, solar and nuclear fuel produces zero carbon dioxide emissions at the point of generation. See our tariff table for more information.

Other environmental benefits:
Other suppliers include the funding of other carbon reducing initiatives such as tree planting in the price of their tariffs. Whilst our GoElectric tariffs don’t directly fund or offer any additional environmental benefits beyond being sourced from renewable generators, EDF is Britain’s biggest generator of zero carbon electricity and as part of the EDF Group (which, in 2017, was the largest generator of renewable electricity in Europe) is committed to going beyond the requirements of 2°C trajectory set by COP21 by drastically reducing our CO2 emissions.

COP26: Is hope an option?

July 30, 2021

Click for  larger version. The graph shows the Mauna Loa record of atmospheric carbon dioxide since 1959 in black. The dotted lines are extrapolations of the trend from each decade, the 1960s, 1970s etc. Also shown are the dates meetings of the COP – the Conference of Parties to the UN framework convention on climate change and the dates of the scientific assessment reports.

Friends, at the end of this year the UK will host ‘COP26’ in Glasgow. And a recent e-mail from a friend in California caused me to ask myself this question:

  • Did I genuinely feel even a scintilla of hope that COP26 would mark a turning point of any kind in our attempts to stop global warming?

After reflection, my answer was – genuinely and sadly – “No”. Please allow me to explain.

Reasons for despair

This reflection was initiated by a particular slide in a presentation (130 Mb pdf available here & Peter Wadhams. TEDX talk here). I have reproduced the essential features in the graph at the top of the page.

It shows the monthly averages of measurements of atmospheric carbon dioxide concentration at the Mauna Loa laboratory in Hawaii since 1959 – the so-called ‘Keeling Curve‘. The data shows seasonal wiggles, but also a continuous rising trend.

Every ppm rise corresponds to roughly 7.8 billion tonnes of carbon dioxide lingering in the atmosphere.

I have fitted a straight line to the data from each decade – 1960’s 1970’s etc, and then extrapolated these lines to 2040.

These extrapolations make clear that not only has the concentration of carbon dioxide in the atmosphere been increasing, but also the rate at which it has been increasing has also increased decade on decade.

  • In the 1960s, carbon dioxide concentrations were increasing at 0.77 ppm/year.
  • By the 2010s, carbon dioxide concentrations were increasing more than three times faster, at 2.37 ppm/year.
  • The only decade in which there was essentially no acceleration in the levels of carbon dioxide over the previous decade was the 1990’s. This was reportedly due to the chaos which followed the collapse of the Soviet Union.

Also shown are the dates of the 25 previous Conferences of the Parties (COPs) to the United Nations Framework Convention on Climate Change (UNFCCC).

Additionally shown are the dates of the six IPCC Assessment Reports summarising the state of scientific knowledge about Climate Change.

Looking at these data together offers a sobering perspective.

If one were feeling uncharitable, one might argue that the previous 25 COPs appear to have made no difference whatsoever to the growth in atmospheric carbon dioxide concentration.

And one might then conclude that a priori, COP26 would be similarly unlikely to make a difference. If one then recalled that the diplomatic wizard Boris Johnson was hosting the event, one’s hopes might fall yet further.

And if you feel despair or anger then I think these are perfectly understandable responses.

But in fact I think the previous COPs have had an effect. But the fact that they have failed to stop further increases in carbon dioxide emissions – is testament to the difficulty and complexity of the challenge we all face.

Reasons for hope

Although I despair that COP26 of the UNFCCC will make any progress, I am not without hope. Progress is being made on many fronts.

  • Electricity generation is being transformed by cheap and abundant renewable solar and wind generation.
  • Transport is being electrified at a rate I would not have imagined possible.
  • Renewable energy technologies for space and water heating both domestically and industrially are available off-the-shelf.
  • Consciousness of the need for changes to what we eat and the concomitant changes in agriculture has never been higher.

Some of these developments are local to the UK, but many are global, involving re-deployment of tens of billions of pounds of capital every year.

I would wish for more and faster, but it seems to me that real change has begun.

However the connection between the years of activity at the UN COPs and action visible on the ground is sometimes not clear – but I think it is there.

  • In the UK, solar and and wind generation were not initially cheaper than gas and coal. The electricity they generated was subsidised for many years, and the justification for this was that we needed to reduce carbon dioxide emissions. And our participation in the UNFCCC process was the political backstop that allowed a policy which increased electricity prices.
  • The electrification of road transport is occurring now because of the existence of Elon Musk. Corporations such as GM and Nissan had vastly more resources and time but were corporately unable to give up their profits from ICE vehicles. Once Musk showed that electric cars would be better than ICE cars in every respect, the corporations responded in fear. But despite this striking triumph of free enterprise, government backing for this change has been essential. So-called ‘carbon credits’ from other car companies formed a critical income for the Tesla in its critical early years. And the political framework behind this subsidy was the UNFCCC process.
  • On the ground, almost every dwelling in the UK will require some kind of modification. And the most powerful agents of this change will plumbers and builders who I expect will be thin on the ground at COP26. However the scientists and politicians who will attend COP26, will create policies (such as banning new gas boilers) that will allow manufacturers to invest money to develop new products (such as heat pumps and insulation) that will allow builders and plumbers to effect change on the ground.
  • The consciousness of the need for change is hard to gauge, but I think in the last 20 years it has been enormous. Part of this comes from the scientific work (paid for from public funds) that has made clear the reality of Climate Change and the impact of activities such as farming. This has generated public concern at a level that allows politicians to make unpopular choices – such as raising taxes on particular activities or products.

So despite the unpromising trend in the graph at the head of this article, and despite my very low expectations of COP26. I am still not entirely despairing.

In the end, climate change is a threat to every country on Earth and the UN processes provide a framework – albeit highly imperfect – for humanity to act. Ultimately, a world where COP26 (and 27 and 28…) takes place offers more possibilities for change than a world without those meetings. And this is true even if a particular meeting is disappointing.

My hope

My hope is that before I die I will  look up the data from Mauna Loa and see a reduction in the slope of the curve.

So that when I extrapolate the data from the 2020s, it will be shallower than the slope from the 2010s.

I daren’t hope to see the curve flatten, but I hope my children will live to see that, and then eventually to see it fall.

This would still commit us to a great deal more Climate Change in the coming century, much of it which will be very bad.

But to have collectively and deliberately changed the slope on the Keeling curve would be a sign to all humanity that we have begun to take care of our own planet. And that the age of fossil fuels was ending.

Back Down to Earth

June 22, 2021

Friends, at the end the last article I wrote:

The combination of 12 solar panels and a Tesla Powerwall battery has been sufficient for us to be practically off-grid for the last 3 months. And that will probably continue for another 3 months.

..it feels astonishing to be sustaining a good quality of life powered entirely by the Sun.

As we approach the summer solstice, I feel like I have reached apogee in a solar-powered rocket, and I am briefly floating weightless.

A week of miserable weather has brought me firmly back down to Earth.

After 87 days drawing no electricity from the grid, as the chart below shows, we have had to re-connect.

Click for a larger version. The graph shows daily electricity drawn from the grid (kWh) since November last year. After the battery installation, this fell to almost zero. Also shown is daily electricity used from the battery and solar panels (kWh). This has risen recently because electricity is now being used for air conditioning, cooking and domestic hot water.

We have now switched the mode of operating the battery so that it charges itself at night using off-peak electricity.

Solar Statistics: Summer Solstice Review

The summer solstice is probably a good point to review the performance of the solar panels installed last November 2020.

The £4200 system consists of 12 Q-Cells Duo BLK-G8 panels tilted at 40°. Six panels facing 25° East of South and six facing 65° West of South. A fuller description can be found here.

Click for a larger version. The graph shows daily solar generation (kWh) versus day of the year along with a 5-day running average. Also shown are two estimates for expected generation (kWh)alongside typical daily consumption.

The last 5 days have seen very poor generation. Last Friday 18th June, generation was just 2.3 kWh – more typical of mid-winter than mid-summer! And a battery with 13.5 kWh capacity is not big enough to see us through this dip.

Click for a larger version. The graph shows cumulative solar generation (kWh) versus day of the year along with a cumulative exports (kWh). Also shown are lines showing the estimated annual and semi-annual generation as specified by the installer.

Total generation so far this year is 1780 kWh – very close to 50% of the installer’s annual estimate.

The system has exported 590 kWh, my benevolent contribution to the grid, and I have used around 1200 kWh saving me around £250 compared to the situation without solar panels and batteries. If the panel’s performance is similar in the second half of the year, this would give a modest 3.5% return on my investment.

Carbon dioxide emissions 

Some fraction of this generation will have displaced gas generation which would have given rise to 0.45 kgCO2 per kWh, and some fraction will have displaced a typical generating mix which would have given rise to roughly 0.2 kgCO2 per kWh.

So depending on the assumptions made, my electricity generation has probably avoided emissions of between 350 kg and 800 kg of carbon dioxide so far this year, and will probably have avoided between 0.7 and 1.6 tonnes of CO2 by the end of the year.

The bigger plan 

The installation last week of the Air Source Heat Pump, a Vaillant Arotherm plus 5 kW model, together with a domestic hot water cylinder, marks the end of my investments in reducing carbon dioxide emissions from the house.

The ‘magic’ of the heat pump is that it uses 1 kWh of electrical energy to extract typically 2 kWh of thermal energy from the air, yielding around 3 kWh of heating.

This is central to reducing my carbon dioxide emissions. It has allowed me to replace the polluting gas boiler.

To compare carbon dioxide emissions with what what would have happened if I had made no changes, I have made a month-by-month estimate of household carbon dioxide emissions over the next 20 years.

These calculations are still preliminary, but the figure below shows their general form. It charts the anticipated carbon dioxide emissions if I had done nothing, alongside the anticipated carbon dioxide emissions in my plan.

Click for a larger graph. This chart shows month-by-month calculations of anticipated household carbon dioxide emissions based my current plan, or the do nothing alternative.

The green line shows an initial rise due to the 10.5 tonnes of carbon dioxide emitted during the manufacture of:

  • External Wall Insulation Boards (1.6 tonnes)
  • External Wall Mortar (1.0 tonnes)
  • Argon Triple Glazing (1.9 tonnes)
  • Solar Panels (1.6 tonnes)
  • Battery (1.4 tonnes)
  • Heat Pump (1.5 tonnes)
  • Air Conditioning (1.5 tonnes)

The green line then shows a much lower slope. The calculations indicate a break-even in terms of carbon dioxide by the end of 2023, and the non-emission of around 60 tonnes of carbon dioxide by 2040 when compared with the ‘do nothing‘ alternative.

So…

It’s disappointing to be back ‘on grid’ for a few days, but overall the solar panels are performing pretty much as anticipated, already avoiding the emissions of hundreds of kilograms of carbon dioxide.

And they are just one part of the plan. The installation of the Air Source Heat Pump is the last part of the plan, and I will now monitor the house to see if my expectations are fulfilled.

Floating

June 16, 2021

Friends, I am experiencing a sensation akin to weightlessness.

It is the feeling of living a good life without consuming electricity or gas from the grid.

Electricity

The combination of 12 solar panels and a Tesla Powerwall battery has been sufficient for us to be practically off-grid for the last 3 months. And that will probably continue for another 3 months.

Click for a larger version. The graph shows daily electricity drawn from the grid (kWh) since November last year. After the battery installation, this fell to almost zero. Also shown is daily electricity used from the battery and solar panels (kWh). This has risen recently because electricity is now being used for air conditioning, cooking and domestic hot water.

Household use of electricity has gone up in recent days as we have used electricity for water heating and air conditioning. But we have plenty to spare at the moment.

We have exported around 580 kWh so far this year, displacing mainly gas-powered generation. So in addition to not emitting any carbon dioxide from our own home, we have reduced emissions from other people’s homes by around 200 kg.

Gas

Last week we had the gas boiler removed and so since then we have only been using gas for cooking.

But on some days, the cooking load has been light and all we need to do is cook a bit of rice which can be done easily on a single ring induction heater (similar to this).

And so on those days, we have drawn no gas from the network.

Floating

At the moment it is just for a few days at a time. But it feels astonishing to be sustaining a good quality of life powered entirely by the Sun.

In fact, because of the air conditioning, I am probably cooler (thermally, not style-wise) than you.

As we approach the summer solstice, I feel like I have reached apogee in a solar-powered rocket, and I am briefly floating weightless.

Heat Pumps#1

June 10, 2021

Installation of an air source heat pump (ASHP) in my own house is sadly on hold while the installers await delivery of a part. So I thought I would take this opportunity to update you with one or two things I have learned about how real-world heat pumps operate.

What is a heat pump?

I am preparing an article about how heat pumps work internally, but considering only their operational behaviour, they work like the device illustrated below.

Click for a larger version.

  • Powered by electricity, they extract heat from the air.
  • Cold water enters the the heat pump.
  • Warm water flows out.

The engineered ‘miracle’ of a heat pump is that 1 kWh of electrical energy can extract between 2 kWh and 4 kWh of heat energy from the air.

It might seem that nothing could be simpler or more wonderful? But the engineering reality behind the ‘miracle’ requires that the heat pump be operated carefully.

The problem

The key problem is that heat pumps require a high flow of water through them in order to enable efficient operation of the heat exchangers which extract heat from the air. Typical flows are in the range 20 to 40 litres per minute of water.

For my 5 kW heat pump, this can warm such a flow of water by only 2 or 3 °C. So how can such a device heat water to 55 °C?

Domestic Hot Water

When the heat pump is configured to heat domestic hot water – for sinks and bathrooms – then the circuit looks like the figure below.

Click for a larger version. Schematic diagram of how a heat pump heats domestic hot water. See text for further details.

In DHW-mode, the water in the heat pump circuit is passed through a steel tube wound inside an insulated water storage cylinder. This acts as a heat-exchanger between the water in the heat pump circuit, and the water in the cylinder.

But remember, the ‘hot’ water in the heat pump circuit is just a degree or two warmer than the returning ‘cold water. So how can this ever heat the domestic water to 55 °C.

The trick is having a smart heat-pump controller and low losses in the connecting pipework.

The heat pump controller first sets the heat pump operating parameters to warm the water returning from the DHW by a few degrees.

As the DHW tank warms, the returning water also warms, and the controller slowly adjusts the operation of the heat pump to increase the temperature of the water it supplies to the DHW tank. Eventually the controller detects when the water in the DHW tank has reached its set temperature.

So for example, if the outside temperature is 10 °C, and the water returning from the DHW tank is initially at 20 °C, then:

  • Initially the controller configures the heat pump to heat the flowing water to (say) 22 °C. Pumping heat from air at 10 °C to water at 22 °C can be done much more efficiently than pumping heat from 10 °C to 55 °C.
  • At first the temperature of the water returning from the DHW tank will be only slightly above 20 °C. But as heat is transferred to the DHW tank the temperature of the water returning from the DHW tank increases.
  • In response to this increase in the temperature of the returning water, the controller re-configures the heat pump to an incrementally higher temperature.

By adopting this clever strategy:

  • The first part of the heating can be done with higher efficiency – perhaps resulting in 4 units of heating for each unit of electrical work.
  • The later part of the heating is less efficient and might only results in 3 units of heating for each unit of electrical work.
  • So overall – depending on the maximum temperature required – the so-called coefficient of performance (COP) is usually somewhere between 3 and 4.

Space Heating 

When the heat pump is configured for room heating – so called ‘space heating’ in the lingo – then the circuit looks like the figure below.

Click for a larger version. Schematic diagram of how a heat pump heats radiators. See text for further details.

I was surprised to find that in this mode of operation the water from the heat pump is not passed directly through the system of radiators.

Instead, most of the water passes through a short section of tubing called a ‘low loss header’ and goes straight back to the heat pump. This allows the heat pump to operate at high flow rates.

The water used in the radiators is drawn from the top of the ‘low loss header’ and returns – cooler – to the top of the ‘low loss header’.

However there is almost no pressure difference between the top and bottom of the ‘low loss header’ – and so very little water would naturally flow through the radiators. So a hydraulic pump is used to push water through the radiators.

The cooled water from the radiators now mixes with the main flow at the bottom of the ‘low loss header’ and returns to the heat pump.

Click for a larger version. Schematic illustration of a ‘low loss header’ See text for further details.

So for example, if the heat pump is supplying 20 litres per minute of water at 55 °C to the ‘low loss header’:

  • The hydraulic pump draws perhaps 4 litres per minute of water at 55 °C leaving 16 litres per minute to flow straight through the header.
  • The return water from the radiators is cooled to (say) 45 °C.
    • From this one can calculate that the radiators have provided heating of 2.8 kW.
  • So at the bottom of the ‘low loss header’ there is a mixture of:
    • 16 litres per minute of water at 55 °C
    • 4 litres per minute of water at 45°C
    • When mixed together this makes 20 litres per minute of water at approximately 53 °C which is returned to the heat pump.

At first I was puzzled by this arrangement, but then I realised it was clever trick.

  • It allows the heat pump to operate at high flow rates and yet heat water only over small temperature differences.
  • And it allows the radiators to operate with lower flows and bigger temperature drops.

For those with experience of electronics, it is analogous to the ‘impedance matching’ effect of a transformer.

It’s complicated…  

Things are more complicated than these diagrams would suggest.

Firstly, the heat pump can only operate in one mode at a time.

So the heat pump controller changes modes by operating a valve to direct the water from the heat pump either to the DHW storage tank or the radiators.

Secondly, there are numerous features incorporated for reasons of safety or maintainability.

Some of these guard against the effects of thermal expansion of the water, some guard against the (low risk) of Legionella infection, and some are filters or energy monitoring components.

But I hope the explanations above come close to getting to the gist of heat pump operation.

I have lots more to say about heat pumps: so stay tuned!

Gas Boilers versus Heat pumps

May 18, 2021

Click for a larger version. A recent quote for gas and electricity from Octopus Energy. The electricity is six times more expensive than gas.

We are receiving strong messages from the Government and the International Energy Agency telling us that we must stop installing new gas boilers in just a year or two.

And I myself will be getting rid of mine within the month, replacing it with an Air Source Heat Pump (ASHP).

But when a friend told me his gas boiler was failing, and asked for my advice, I paused.

Then after considering things carefully, I recommended he get another gas boiler rather than install an ASHP.

Why? It’s the cost, stupid!

Air Source Heat Pumps:

  • cost more to buy than a gas boiler,
  • cost more to install than a gas boiler,
  • cost more to run than a gas boiler.

I am prepared to spend my own money on this type of project because I am – slightly neurotically – intensely focused on reducing my carbon dioxide emissions.

But I could not in all conscience recommend it to someone else.

More to Buy

Using the services of Messrs. Google, Google and Google I find that:

And this does not even touch upon the costs of installing a domestic hot water tank if one is not already installed.

More to Install

Having experienced this, please accept my word that the installation costs of an ASHP exceed those of replacing an existing boiler by a large factor – probably less than 10.

More to Run

I have a particularly bad tariff from EDF,  so I got a quote from Octopus Energy, a popular supplier at the moment,

They offered me the following rates: 19.1 p/kWh for electricity and 3.2 p/kWh for gas.

Using an ASHP my friend would be likely to generate around 3 units of heat for every 1 unit of electricity he used: a so-called Coefficient of Performance (COP) of 3.

But electricity costs 19.1/3.2 = 6.0 times as much as gas. So heating his house would cost twice as much!

More to buy, install and run and they don’t work as well!

Without reducing the heating demand within a house – by insulation – it is quite possible that my friend would not be able to heat his house at all with an ASHP!

Radiator output is specified assuming that water flowing through the radiators is 50 °C warmer than the room. For rooms at 20 °C, this implies water flowing at 70 °C.

A gas boiler has no problem with this, but an ASHP can normally only heat water to 55 °C i.e. the radiators would be just 35 °C above room temperature.

As this document explains, the heating output would be reduced (de-rated to use the correct terminology) to just 63% of its output with 70 °C flow.

What would you do?

Now consider that my friend is not – as you probably imagined – a member of the global elite, a metropolitan intellectual with a comfortable income and savings. I have friends outside that circle too.

Imagine perhaps that my friend, was elderly and on a limited pension.

Or imagine that they were frail or confused?

Or imagine perhaps that they had small children and were on a tight budget.

Or imagine that they were just hard up.

Could you in all honesty have recommended anything different? 

These problems are well known (BBC story) but until this cost landscape changes the UK doesn’t stand a chance of reaching net-zero.

 

Air Conditioning versus Air Source Heat Pump

May 15, 2021

Click for a larger version. Similarities and differences in how an air source heat pump (ASHP) or an air conditioning (AC) system warms a home. All the components inside the dotted green line are contained in the external units shown. A key design difference is whether or not the working fluid is completely contained in the external unit. See text for more details.

Regular readers will probably be aware that – having reduced the heating demand in my house – my plan is to switch away from gas heating and install an electrically-powered air source heat pump to heat the house and provide domestic hot water.

But next week I am also installing air conditioning, something which is traditionally not thought of as very ‘green’. What’s going on?

Why Air Conditioning?

I have two reasons.

My first reason is that, as you may have heard, the whole world is warming up! Last year it reached 38 °C in Teddington and was unbearably hot for a week. I never want to experience that again.

During the summer the air conditioning will provide cooling. But assuming the heating comes with good weather, the air conditioning will be totally solar powered, and so it will not give rise to any CO2 emissions to make matters worse!

My second reason is that in the right circumstances, air conditioning is a very efficient way to heat a house. That’s what this article is about.

Heat Pumps

Air Conditioners (AC) and Air Source Heat Pumps (ASHP) are both types of heat pumps.

In scientific parlance, a heat pump is any machine that moves heat from colder temperatures to higher temperatures at the expense of mechanical work.

Note: to distinguish between the general scientific idea of a heat pump, and the practical implementation in an air source heat pump, I will use abbreviation ASHP when talking about the practical device.

The general idea of a heat pump is illustrated in the conceptual schematic below.

As shown, the pump uses 1 unit of mechanical energy to extract two units of heat energy from air at (say) 5 °C and expel all 3 units of energy (1 mechanical and 2 thermal) as heat into hot water at (say) 55 °C.

Click for a larger version. Traditional representation of the operation of heat pump.

Heat pumps can seem miraculous, but like all good miracles, they are really just applied science and engineering.

A heat pump is characterised using two parameters: COP and ΔT.

  • A heat pump which delivers 3 units of heat for 1 unit of work is said to have a coefficient of performance (COP) of 3.
  • The temperature difference between the hot and cold ends of the heat pump is usually called ‘Delta T’ or ΔT.

Obviously engineers would like to build heat pumps with high COPs, and big ΔTs and they have used all kinds of ingenious techniques to achieve this.

But it turns out that heat pumps only operate with high COPs when the ΔT is small and when the heating power is low. There are two reasons.

  • Firstly, the laws of thermodynamic set some absolute limits on the COP achievable for a given ΔT.
    • Most practical heat pumps don’t come close to this thermodynamic limit for a variety of mundane reasons.
    • The maximum COP for moving heat from 5 °C to 55 °C is 6.6.
    • The maximum COP for moving heat from 5 °C to 20 °C is 19.5.
  • Secondly, in order to heat a room to (say) 20 °C, the hot end of the heat pump needs to be hotter than 20 °C.
    • Typically the hot end of the heat pump must be 5 °C to 10 °C warmer than the room in order that heat will flow out of the heat pump.
    • Additionally the cold end of the heat pump must be 5 °C to 10 °C colder than the external air in order that heat will flow into the heat pump.
    • The interfaces between the ends of the pump and the environment are called heat exchangers and designing ‘good’ heat exchangers is tricky.
    • A ‘good’ heat exchanger is one that allows high heat flows for small temperature differences.

So now we have seen how heat pumps are characterised, let’s see how heat pumps are used domestically.

Air Source Heat Pump (ASHP) versus Air Conditioner (AC)

The schematic diagrams  below show how a house is heated by an ASHP and an AC system. Both systems operate using a working fluid such as butane, which is ingeniously compressed and expanded. The details of this process are not the topic of this article so here I am glossing over the fascinating details of the device’s operation. Sorry.

Click for a larger version. How an air source heat pump (ASHP) warms a home. All the components inside the dotted green line are contained in the external unit shown. A key design feature is that the working fluid is completely contained in the external unit and heat is transferred to the central heating water by a heat exchanger.

Click for a larger version. How an air conditioner (AC) warms a home. All the components inside the dotted green lines are contained in either the external unit or the fan coil unit shown. A key feature is that the working fluid itself flows into the fan coil unit and heats the air directly.

We can compare the operation of the two systems in the table below.

Air Conditioner Air Source Heat Pump
Air at (say) 5 °C is blown over a heat exchanger and evaporates the working fluid.

 

The same.
The working fluid is then compressed – that’s the bit where the work is done – and liquefies, releasing the captured heat.

 

The same.
The hot working fluid – now at ~30 °C then flows through a pipe to an indoor heat exchanger (fan coil unit) where air is blown over the pipe and heated to 20 °C. The hot working fluid – now at ~60 °C then flows through a heat exchanger and transfers the heat to water in my central heating system at ~55 °C
No corresponding step  

The 55 °C water then flows through a radiator in my room, heating the room by radiation and by convective heat transfer to air at ~20 °C.

Looking closely at the figures and table above, one can see that the operation of the ASHP and the AC system are broadly similar.

However the ASHP has to operate with a bigger ΔT (~55 °C versus ~25 °C) than the AC system, and also has to transfer heat through an extra heat exchanger.

Both these factors degrade the achievable COP and so for my application, the specified COP for an ASHP is just over 3, but for the AC system, it is just over 5.

In my well-insulated house, when the external temperature is 5 °C, I require typically 36 kWh per day of heating, equivalent to 1.7 kW continuous heating. I can achieve this in several ways:

  • Using gas I must burn ~40 kWh of gas at 90% efficiency costing 40 x 3p (£1.20) and emitting 40 x 200 g = 8 kgCO2
  • Using an ASHP with a COP of 3, I must use ~36 kWh/3 = 12 kWh of electricity costing 12 x 25p (£3.00) and emitting 12 x 200 g = 2.4 kgCO2
  • Using an AC system with a COP of 5, I must use ~36 kWh/5 = 7.2 kWh of electricity costing 7.2 x 25p (£1.80) and emitting 7.2 x 200 g = 1.4 kgCO2
  • Using a domestic battery and buying the electricity at night for 8p/kWh, I can reduce the cost of using an ASHP or AC system by a factor of 3 to £1.00/day or £0.60/day respectively.

[Note: In these calculations I have assumed that the carbon dioxide emissions per kWh are same for both gas and UK electricity (200 gCO2/kWh) which is roughly correct for 2021]

So using an AC system I should be able to achieve domestic heating with lower carbon dioxide emissions than an ASHP.

My plan

In my case I need to heat water for my home to 55 °C for use in showers and basins. So I need an ASHP for that. And since I already have radiators in every room, hooking up the ASHP to the radiator circuits is smart double use.

The AC system I am having installed will have 1 external unit and 2 internal ‘fan coil units’. One unit will be in my bedroom (a sheer indulgence) and the other will be high up on the stairs, allowing air to be either blown down to the ground floor where I hope it will circulate, or blown towards the bedrooms.

My hope is that, when used together, the AC system (COP~5) will reduce the heating output required from the radiators so that I can reduce the flow temperature of the water from 55 °C to perhaps 40 °C. This reduces their heat output, but increase the COP of the ASHP from 3 to perhaps 4.

The main difficulty that I foresee is the extent to which the AC heating will actually permeate through the house and so reduce the amount of heating required by the ASHP.

So I am not sure how much heating will be required by the ASHP acting through the radiators, and whether the radiators will work at low flow temperatures. It is possible I might need to replace a few radiators with ones which work better at low temperatures.

It is not at all obvious that this plan will actually work at all – but I think it is worth a try.

Kit

The air conditioning I am having installed is a Daikin 2MXM40 multi-split outdoor unit with two FTXM25 indoor air units. (Brochure)

The model of heat pump I will have installed is a Vaillant Arotherm plus 5 kW. It can supply up 5 kW of heating at 55 °C with a COP of 3  – i.e. it will use just 1.6 kW of electrical power to do that – and heat water to 55 °C. Water storage will be a 200 litre Unistor cylinder. A brochure with technical details can be found here, and a dramatic video showing the kit is linked at the end of this article.

When I have come to terms with how much money I am spending on this, I will share that information. But at the moment it hurts to think about it!

Anyway: the adventure begins next week!

 

ICE vs BEV vs FC

May 8, 2021

Friends, forgive my titular obscurantism. This is an article about how to compare the efficiencies and carbon emissions of cars of different types:

  • ICE = Internal Combustion Engine i.e. petrol and diesel cars
  • BEV = Battery Electric Vehicles
  • FC = Fuel Cell cars

A cloud over Teddington!

While relaxing at de Podesta Towers yesterday, a momentary cloud of worry passed through the otherwise blissfully blue sky of my retirement. I thought: how do you compare the efficiencies of these different classes of vehicles?

  • ICEs specify miles per gallon or litres of fuel per 100 km.
  • BEVs specify how many kilowatt hours (kWh) of energy are required to travel 100 km.
  • FCs specify how many kilograms of hydrogen (kgH2) are required to travel 100 km.

Comparing CO2 emissions

One way to compare these disparate types of vehicles is to compare their carbon dioxide emissions.

For all vehicles there are both proximate emissions which occur as the vehicle is driven, and source emissions associated with the charging – in the general sense – of the vehicle.

This categorisation is sometimes succinctly summarised as

  • well-to-tank emissions,
  • tank-to-wheels emissions,

which together add up to make well-to-wheels emissions.

  • For ICEs there are both well-to-tank emissions and tank-to-wheels emissions.
  • For BEVs ‘well-to-tank’ emissions occur at the power stations used to generate the electricity that charged the battery. There are no proximate or tank-to-wheels emissions.
  • For FCs, ‘well-to-tank’ emissions occur at the power stations used to generate the electricity used to separate and compress the hydrogen gas. There are no proximate or tank-to-wheels emissions.

For ICEs, the CO2 emissions per kilometre are a multiplier of the fuel efficiency, with different factors for petrol or diesel, plus a factor for the emissions associated with the preparation of the fuel.

  • Factor for creation of the fuel
  • Vehicle specification in Litres per 100 km
  • Factor for fuel type: petrol or diesel

For BEVs the the CO2 emissions per kilometre are the product of the three factors:

  • CO2 per kWh of the charging electricity – the so-called carbon intensity of the electricity.
  • The efficiency of the charging.
  • Vehicle efficiency specified as kWh per 100 km

For FCs the the CO2 emissions per kilometre are the product of the two factors:

  • CO2 per kWh of the electricity used in the electrolysis and compression of the H2 gas.
  • Vehicle efficiency specified as kgH2 used per 100 km

So lets look at some examples.

Toyota Mirai FC 

  • Specifications suggest an average of 0.75 kgH2/100 km
  • Wikipedia suggests Electrolysis at 80% efficiency and compression together take 65 kWh/kgH2
  • Current UK carbon intensity is 200g/kWh which is expected to fall to 100g/kWh in 2030.
  • Multiplying we find CO2/km = 0.75 [kgH2/100 km] x 65 [kWh/kgH2] = 49 kWh/100 km
  • Multiplying by the carbon intensity x 200 [g/kWh] = 97 gCO2/km in 2021 falling to 49 gCO2/km in 2030

Perhaps you can see why my brow furrowed over trying to figure this out!

Tesla Model 3 BEV

  • Specifications suggest 26 kWh/100 km
  • Charging efficiency is typically 90% i.e. if we use 28.6 kW of grid electricity to charge the car, 90% of that (26 kWh) will be stored in the battery.
  • Multiplying by the carbon intensity of 200 [g/kWh] = 57 gCO2/km in 2021 falling to 28 gCO2/km in 2030

It is hard to pick a single ICE car comparable with these rather premium BEVs and FCs. I have looked through the Mercedes A-class brochure which states that under the Worldwide Harmonized Light Vehicles Test Procedure (WLTP) the proximate CO2 emissions are in the range

  • 130 to 150 gCO2/km
  • This corresponds to about 6 litres/100 km or 47 miles per UK gallon
  • The above figures are only tank-to-wheels emissions. The well-to-tank emissions amount to roughly 36 gCO2/km (link)

Click for a larger version. Summary of the amount of carbon dioxide emitted per km of travel for a fuel call car (Toyota Mirai), a battery electric vehicle (Tesla Model 3), or a premium internal combustion engined car (Mercedes A Class). In the UK grid electricity is expected to have a lower carbon intensity by 2030 which should reduce the emissions associated with BEV and FC cars. This chart ignores any embodied carbon dioxide in the construction of the vehicles.

So in terms of carbon dioxide emissions, a BEV is likely to be better than an FC car and both are much better than comparable ICE cars.

BEV vs FC

The reason that the FC car is so much poorer than the BEV is because of the complexities of first obtaining hydrogen, compressing it (link), and then converting the chemical energy back to electricity in the fuel cell.

The advantages of fuel cell cars over BEVs (faster re-filling and longer ranges) are slowly being eroded by advances in battery technology. And  given the paucity of hydrogen filling stations, I think FC cars will prove to be a historical dead end.

BEV vs ICE: Efficiency

The key to the success of ICEs has been the astonishing energy density of their fuels.

Click for a larger version. This graphic (modified Wikipedia graphic) shows the energy density (kWh/litre) versus specific energy (kWh/kg) for various fuels. Strikingly, batteries have very low energy density and specific energy than ICE fuels. Note also the very high specific energy density of hydrogen (per unit mass), but the the very low energy density (per unit volume).

As the chart above shows the energy density of either petrol or diesel – whether expressed per kilogram or per litre – is way more than that of lithium ion batteries used in BEVs.

If we average gasoline (12.9 kWh/kg, 9.5 kWh/l) and diesel (12.7 kWh/kg, 10,7 kWh/l) we get a rough figure for liquid fuels of 12.8 kWh/kg and 10.1 kWh/l.

In contrast the figures for a lithium ion battery are in the range 0.1 – 0.24 kWh/kg and 0.25–0.73 kWh/l, smaller than liquid fuel by factors of at least 53 for per unit mass and 14 per unit volume.

It might seem that there would be no way that batteries could ever compete. But in fact ICEs are profilgate with their energy use.

ICEs only extract about 35% of this chemical energy as mechanical energy and then frictional losses in the engine and drivetrain reduce this to about 20%. So that makes the advantage factors of roughly 10 per unit mass and 2.8 per unit volume.

In contrast, after taking account of regenerative braking, BEVs can convert about 90% of their stored energy to motive power.

This still leaves ICEs with an advantage and to compete BEVs end up with a heavy battery which takes a large volume. And it can’t quite be made big enough to challenge the range of ICEs.

But it seems that BEVs have become ‘good enough’.

ICEs are very highly evolved with more than 100 years of continuous development, but lithium ion batteries are still relatively new and are likely to continue to get incrementally better and cheaper. And as the carbon intensity of the grid reduces over the coming years the cars will become greener still.

Blue sky 

And thus the cloud passed, and the sky cleared, and life at de Podesta Towers returned to its previous untroubled pace.

 


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