Lying in bed one night, I woke up thinking about the portable air conditioner. This is the sort of thing that apparently occupies my mind in the small hours.
One inlet and 2 outlets.
At least, that was how it appeared. Cold air came back into the room through one outlet. Hot air went outside through the other. "Hang on. If air was being pumped out of the room, where was its replacement coming from?"
More accurately, the machine has 2 air circuits, both drawing air from the room, but only one returning it. One stream passes over the cold evaporator and comes back into the room. The other passes over the hot condenser and is sent outside through the exhaust hose.
That air has gone.
The room does not collapse like a crisp packet. Its pressure falls by a tiny amount, which is enough to pull replacement air in under the door, around windows, through vents and through the various small gaps in the building.
During a heatwave, that replacement air is warm. If it is humid, the machine then has to remove that moisture as well. So the machine cools the room while creating the pressure difference that draws more warm air into it. It is partly battling itself.
I have dealt with the most obvious problem. The exhaust hose passes through a properly cut hole in a Perspex panel covering the window, so the hot exhaust cannot simply curl around and come straight back in through the same opening. The Perspex does that job perfectly well. What it cannot do is replace the air being pumped outside. That still has to come from somewhere else.
The hose is not helping much either. It carries the hottest air in the system and runs through the room I am trying to cool. It gets hot and gives some of that heat back before the air reaches the window. Keeping it short helps. Insulating it would help more. Neither alters the fact that the hot side of the air conditioner is sitting in the bedroom.
The machine does work. Ours makes a considerable difference in the large upstairs bedroom, particularly when the outside temperature gets into the 30s. It is simply a rather compromised way of doing the job.
Then there are the little plug-in air conditioners now infesting online adverts. Most are fans blowing air across a wet pad. They may cool the air immediately in front of them, particularly in a dry climate, but they do not remove heat from a room. No amount of breathless advertising can repeal the laws of thermodynamics. During the heatwave, some of them have doubled or tripled in price. The technology has not improved. Only the opportunity has.
A proper split system keeps the indoor and outdoor air circuits separate. The indoor unit cools the room air and returns it to the room. Refrigerant carries the heat outside to the condenser. No room air is expelled, so there is no negative pressure pulling warm replacement air into the house. It does not normally create positive pressure either. It moves the heat outside without throwing the room air out with it, which seems the more sensible arrangement.
A car works in much the same way. The evaporator is inside the cabin and the condenser is outside, at the front of the car. On recirculation, cabin air stays in the cabin. Select fresh air and the blower deliberately brings outside air in, with the excess escaping through vents elsewhere. What the car does not do is cool the cabin, take some of that cooled air, heat it up and throw it out under the bonnet.
We are therefore considering fitting a split system next year, but cooling the air after the heat has entered the house is only part of the answer.
Our internal blinds are pale grey and white, which helps, although the dark curtains are less useful once the sun is pouring through the glass. By then the heat is already indoors and the curtains are merely absorbing it on our behalf.
We are now considering proper Mediterranean-style external shutters as well. Not aluminium screens or fabric blinds, but folding oak louvred shutters to match the oak cladding and oak windows.
They would fold back clear of the outward-opening windows when not needed, then close during the hottest part of the day and stop much of the solar radiation before it reached the glass. That is rather more sensible than letting the sun heat the room and then paying a machine to remove the heat afterwards.
Fortunately, the windows are framed by substantial oak verticals, so the shutters could be fixed directly into something capable of carrying them rather than being screwed hopefully into decorative cladding. They would still need serious stainless-steel hinges, proper hold-backs and enough clearance for the oak to move with the seasons. An oak shutter caught by a gust of wind is not a blind. It is a lever.
The shutters would reduce the heat entering the room in the first place. The split system would then deal with what remained.
The running cost is less of an objection for us because air conditioning is needed most when the sun is out, which is also when our solar panels are producing. The battery can cover some of the gaps. It will not make the cooling free, but much of the electricity should come from the roof rather than the grid.
A split unit can also provide heat in cooler weather, so it would not spend most of the year hanging on the wall doing nothing.
External shutters to keep the heat out. A split system to remove what still gets in. Solar panels producing when the cooling demand is highest.
There is a wider irony here too.
When a heatwave drives up demand for air conditioning, gas-fired power stations are often brought in to supply the extra electricity. But as the air temperature rises, gas turbines themselves lose output. Hotter air is less dense, so the compressor draws in less oxygen by mass and the turbine cannot burn quite as much fuel. At precisely the point when more electricity is wanted, some of the supposedly firm gas capacity begins to ebb away - - by as much as 20% at 35°C on some turbines, compared with their rated output at the standard 15°C test condition.
And while the station is dealing with today’s heat, the CO₂ it emits adds to the warming that is making future heatwaves more frequent and more severe.
The feedback is slower and global rather than immediate and confined to one bedroom, but the resemblance to the portable air conditioner is difficult to miss. Both tackle the immediate heat while also, in their different ways, helping to make the job harder. The portable unit sucks warm air back into the room. The gas station loses output as the air warms and emits the CO₂ contributing to further warming.
Wind and solar have their own limitations, obviously. The wind can drop and the sun eventually sets. Solar panels also lose a little efficiency as they heat up. But neither generates electricity by adding more CO₂ to the atmosphere.
Curiously, weather dependence is treated as a fatal flaw when renewables suffer from it, but scarcely mentioned when fossil-fuel generation is weakened by the heat and helping to create more of it.
Barring an absurd quotation, I suspect that is where we are heading.
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