Wednesday, September 21, 2016

A heater battery

I've been thinking about home temperature management, and the problem as I see it is:
  • We have a huge supply of existing housing stock, with limited options and money for retrofitting improvements
  • People are unlikely to be satisfied with a 13°C house in winter and a 33°C house in summer
  • As we move to a more renewables-based energy grid, the importance of storing energy increases
  • heating/cooling is, for most people, the largest energy user
  • I believe that the burning of firewood in the city should be minimised
  • If we want to use batteries to supply electricity for heating/cooling, we will need an enormous supply, which will be very expensive (in terms of money and energy) putting it out of reach for most people.
So, we need a heating solution that is
  • Relatively cheap
  • Able to store a large amount of energy during times of plenty – I think in winter it needs to store 4 days' heat (during summer, it really only needs 1 or 2 days as solar PV is very productive)
  • Suitable for retrofitting
 I think I have come up with a possible solution.

Heater-cooler design

In Europe, it is possible to buy a storage heater. It is designed to run on off-peak electricity, which it uses to resistively heat a “bank” of iron “bricks” to about 700°C. Then, during the day when heat is wanted, it blows air over the bricks to warm the room.
My idea is a variant of this, except it uses phase change materials to store heat and “coolth” (the ability to cool air).

Phase change materials (PCMs)

You are familiar with phase change materials if you have ever put ice in a cool box. Ice keeps your food cold not (just) because it is cold, but because ice absorbs a lot of heat as it melts. To put it another way, to it takes a lot of heat to turn ice at 0°C to water at 0°C. If you add the same amount of heat again to the 0°C water, you will raise its temperature to 80°C.

My design uses this principle to store large amounts of heat and coolth.

It uses a PCM that melts at about 30°C. This is encased in small containers (eg. 1L soft-drink bottles) with a large surface area, within a tank of water surrounded by insulation.


In winter, heat is put into the tank, which melts the PCM. Because the maximum temperature of the tank only reaches about 50 – 80°C, there are more options for heating it. Instead of using a resistive heater (such as used in the European storage heater) we can use a bank of evacuated tubes or heat pump and get either free heat, or 4 kWh of heat for every 1 kWh of electricity (it might even be possible to use a roof-top solar pool heater). This melts the PCM and stores the heat. Later, when the building occupants want to warm the room, a small fan blows room air over the heated tank and warms the room air.


In summer, a heat pump can be used to pump heat out of the tank. This can freeze the PCM first (at 30°C), and then the water surrounding it (at 0°C). This stores “coolth” and in the evening when people come home from work, they can use the tank to cool the air in the room.


I have done some analysis and modelling in a spreadsheet that can be found here.
I have modelled the heater’s performance under various design considerations. At the moment, here is what I have settled on:
  • Heat/coolth storage volume: 350 L (total size: 1.2 x 0.9 x 0.5 m, allowing for 50mm of insulation all the way around)
  • Volume of hot-melt PCM: 250 L
  • Volume of water: 100 L
This design gives the following (theoretical) performance:
Heat storage (for use in winter): 31.8 kWh equivalent
Coolth storage (for use in summer): 12.5 kWh equivalent (I’ve deliberately designed it with less coolth storage, because solar PV is abundant most days in summer, but it could be adjusted. This also assumes that the heat pump can efficiently freeze the water.)


If the home occupant has solar PV and a reverse cycle system anyway, and if the marginal extra cost of my system is $1500 (my unit would replace the internal component of the split system), then even if the installation cost of an equivalent-performance battery system reduces to about $5000 (about ½ current prices) then my system is about 25% cheaper (this calculation is on the basis of heat/coolth delivered, and considers that it won’t be used in spring/autumn. It allocates batteries as having more value, but only at the same rate as heat/coolth – ie. when my heater is unused). In other words: if the price of batteries halves, my heater is still 25% cheaper.
Despite this, I doubt that this can be successfully produced as a commercial product, for the following reasons:
  • It’s a one trick pony – it can’t store energy for general use, only for heating cooling. Batteries store electricity, which is more generally useful. Batteries also have a lot of “public mindshare” and will be hard to compete against
  • The cost of the PCM is significant. To be sold for $1500, it would need to be made for 1/2 that, which I don’t think is possible. I think a bespoke heater could be made for $1500, but that would be using “scrounged” parts wherever possible, since the retail cost of PCM would be about $1500.
Despite this, I think my system has some real advantages over using batteries to power heating/cooling:
  • Its embodied-energy is very low in comparison with batteries
  • It can be expected to have a very long useful life and is repairable (the only moving part is a fan which is replaceable). I could imagine a unit lasting for decades if well-made.
  • It is (relatively) easy to integrate with evacuated tubes to collect extra heat in winter


Most Australian houses have poor thermal performance. This device allows one to store heat/coolth that can be released when it is wanted. Because there are low temperature gradients (the internal temperature range in the device might not go outside -4 to 40°C) it should be easy to insulate. It is a cheap and easy retrofit (compared with improving the building envelope) that allows people to store large amount of heat/coolth to use as they please and is inherently compatible with renewable energy systems.

Sunday, September 4, 2016

Don't abandon the plebiscite

I am not LGBTI, but I am an ally. I strongly believe that all Australians are better-off with marriage equality, and I hope for a future that is more inclusive and less judgemental. I realise that I can’t understand the discrimination that LGBTI people suffer, but have had a few thoughts about the “debate” and how LGBTI people might strategise to get the outcomes they want.
I’m certainly not arguing that a plebiscite is a good outcome (John Howard's government changed the Marriage Act without a plebiscite -- there's no reason this government can't change it too), but it might be the only way of getting marriage equality during this term of parliament. If that’s the case, I think it should be pursued.
Note that I’m only thinking about the politics here, not whether a plebiscite on marriage equality would actually succeed, or any nastiness it might raise – I’m not very knowledgable on either of those things (and it's not for me to glibly talk about LGBTI people tolerating extra abuse for the greater good).

Thoughts in brief:

Marriage equality is a wedge-issue for the Coalition. The (small-l) liberal (progressives) like Turnbull support marriage equality, while the conservatives oppose it. Therefore, there are internal politics at play within the Coalition, with both the (small-l) liberals and conservatives hoping to use the issue to strengthen their power within the party. Turnbull wants to strengthen his prime-ministership, the conservatives want to replace him with someone like Abbott.

Labor is happy that they have found an issue on which to wedge the Coalition. The Coalition successfully wedged Labor on asylum seekers for two decades, and Labor are happy to pay them back. Because of this, politically, Labor are not in a hurry to resolve this. The longer they can prolong this issue, the more political mileage they get from it. The ideal outcome for Labor would be for this to drag on over several electoral cycles. Labor can keep ineffectively “trying” to get marriage equality to happen, and paint the Coalition as the breakers. If this continued, people for whom this is a big issue (ie. The LGBTI community) could become a captive constituency – they are forced to support Labor as the lesser-evil: they know the Coalition will never do it, so they can’t vote for them, but Labor is therefore not in a rush to do anything either. This would be a disaster for the LGBTI community.

For this reason, I think Labor are only too happy to vote against the plebiscite, especially if they can blame the Coalition and not expend political capital. Given the internal politics of the Coalition, the plebiscite might be the only realistic mechanism to get marriage equality – perhaps Labor don’t really want that, and would rather keep this as a hot-button issue that keeps votes coming to them.

There are people in the Coalition who support marriage equality. Since it is not coalition policy, they can’t vote for it (it would require that they cross the floor, and they would be kicked out of the party). But if there was a plebiscite and voting afterwards, perhaps they could. Labor and Greens defeating the plebiscite will weaken Turnbull’s leadership, and make it likely that he will be replaced by a more conservative leader. Is that what progressives really want?

Imagine that a plebiscite is the only option. It might pass or it might fail. If it fails, little is lost and another attempt can be made in a few years. If it passes and goes to parliamentary vote, a few conservatives in the Coalition might oppose it, but most of Labor will support it, as will many other Coalition politicians. It would easily pass. For this reason, if the lower-house fails to do its job and pass legislation on its own, the plebiscite should stay on the table as a fall-back option.

If it comes to a plebiscite, I think the question could be:
Should the government be able to prevent two adults from marrying?
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