Explainer: thermo-economic comparison of home loadshedding solutions.
Market price is temporary. Thermodynamics is eternal.
Techno-economic analysis has been a mainstay of chemical engineering for as long as the field has existed. Lately though, engineers are getting dumber so we need to take shortcuts. We are, however, getting better at communicating so we are able to justify those shortcuts.
This has given rise to the recent trend of thermo-economic analysis, which is less work. Whereas techno-economic analysis adds up all of the capital and operating costs of different process pathways, thermo-economic analysis only looks at the fundamental thermodynamic constraints on system performance. The main benefit of this approach is that the results are universally-applicable and not tied to any particular set of market conditions, or even to any particular equipment design. This makes thermo-economic analysis superior for long-range planning and performance targeting. It’s also less work, which is nice, and it’s more suitable for taking a broad overview of different process pathways.
In light (excuse the pun) of the fact that South Africans are once more looking for ways to mitigate the lifestyle disruptions of loadshedding, it is worth doing some back-of-the-envelope thermo-economic analysis of different loadshedding solutions.
Bottled gas for cooking and/or water heating
Burning hydrocarbons for heat is what separated human from beast in the first place, so any reversion to that modality feels like a defeat. However, the benchmark modality that we’re comparing to consists of burning hydrocarbons for heat, converting that heat to electricity, sending that electricity a long distance through overhead lines and then turning it back into heat.
Cutting out the middle steps saves on a lot of inefficiencies. South Africa has unusually low efficiency of power production because we use a fleet of outmoded coal-burners rather than the more efficient, cheaper and quicker to build Integrated Gasification Combined-Cycle (IGCC) units employed by those few other countries still committed to coal as an energy source. Those coal burners operate at an efficiency in the region of 40%, compared to the 60% we’d have gotten more cheaply if we’d opted for technology from this century when adding to our fleet. Our grid is also comparatively inefficient in transmitting and distributing power, perhaps as low as 75% overall (perhaps lower, even, but Eskom does not publish an overall grid efficiency). That puts the overall thermal efficiency of putting electricity into your home at a rather dismal 30%. By contrast, most companies delivering gas don’t manage to somehow lose most of it on the way. As a result the overall thermal efficiency of delivering gas is in excess of 90% once all of the energy uses involved in compression and delivery are factored in.
There are still thermal losses at the home; not all of the heat is effectively transferred into the water being heated or into the food being cooked. However, these losses are present for both methods.
Overall, using gas for heat is somewhere in the region of two to three times as energy efficient as using electricity from the grid for the same application. This means that switching to gas will reduce your actual energy footprint by 50%–65%. CO2 footprint is reduced even further, because gas has lower CO2 emissions per unit of energy than coal does.
Scarcely any of the energy is drawn from the grid, so a switch to gas also greatly alleviates your contribution to the general electricity crisis.
Conclusion:
Switching to gas for heat is an enormous improvement over electricity on every metric. Cheaper/lower impact options are available for water heating because of its comparatively low temperature. As long as energy pricing is fairly sensible, you will also save money.
Battery pack, optionally with inverter using grid electricity
A fairly popular approach to loadshedding is to simply store up electricity in a battery pack when it’s available, to use when it isn’t.
This is an intuitively poor overall solution because it is still drawing electricity from the same constrained supplier, so it doesn’t alleviate overall grid pressure at all, nor will it reduce environmental impacts.
In fact, it greatly exacerbates both. The charging of a battery is not an entirely efficient process; some of the energy is lost. Moreover, our grids supply Alternating Current (AC) and, to charge a battery, this has to be converted to Direct Current (DC) using a rectifier unit, with energy losses occurring in that step as well. These performance factors vary considerably between specific products but, for a fairly decent setup that doesn’t break the bank, you can hope that maybe 80% of the electricity you draw will wind up in your battery.
For appliances that can directly utilize DC current from a battery there are small losses on discharge, but most of our appliances require the same AC current our wall sockets put out. This requires an additional step, namely an inverter, which converts DC to AC. Generally an inverter will also be integrated with a transformer to reach the right voltage. The overall efficiency of this step varies as well. Home-scale units available commercially operate somewhere around 60% to 80%.
Another source of inefficiency is charge dissipation on the battery. Intuitively you will be aware of this phenomenon. If you charge your phone fully, switch it off and leave it in your drawer for a couple of weeks, you’ll count yourself lucky if it has enough juice left to switch back on. A home battery pack will, likewise, continuously dissipate charge which will necessarily be constantly replaced by drawing from the grid. How much these losses add up to as an inefficiency is massively variable. Typical charge dissipation is in the range of 15% capacity per month, and this is a constant set of losses that take place the entire time the unit is plugged in. Usage only occurs when loadshedding is actually going on, so external factors massively affect the overall efficiency.
All in all, however, it would be something of a miracle if such setups managed an average overall efficiency of 50%, but we will be generous and assume that as a baseline.
Because the initial energy source for these setups is the same inefficient, high-emissions power production/transmission chain of the national grid, all of its same problems and inefficiencies are carried through.
South Africa’s electricity is among the world’s worst in terms of emissions of CO2, sulphur compounds and particulates. Electricity coming out of an inverter in South Africa is at least twice as bad but actually worse, because of the environmental and energy footprints of their manufacture and disposal.
Conclusion: electricity coming out of an inverter system in South Africa is a genuine candidate for the dirtiest energy in the world. It also substantially increases strain on Eskom’s grid.
People using these systems are polluting your air with CO2, sulphur and particulates and all the other junk that arises from coal-burning, and they’re contributing to the need for loadshedding while also reducing the effectiveness of loadshedding as a means of reducing energy use.
But is it justifiable to punch them? That’s a question for the ethicists.
Solar: thermal and/or photovoltaic
South Africa has remarkable amount of solar energy available. Aside from our reasonable proximity to the equator, much of our population also lives at high altitude in low humidity atmosphere, meaning that there is less obstruction to the sun’s rays than in much of the world. This makes solar energy an obvious candidate for independent energy.
Photovoltaic materials have been getting cheaper and more efficient, but the efficiency of converting sunlight into electricity still tends to fall short of the 20% mark, and the panels still use rare-earth metals that are quite scarce and represent a high environmental impact both in their mining and their eventual disposal. The low-tech cousin, solar-thermal energy, outperforms solar-photovoltaic on these metrics, with higher collection efficiencies at a fraction of the cost, with environmentally innocuous materials of fabrication. A solar water heater bought cheaply off the shelf or cobbled together with bits from a local hardware store can quite happily reach temperatures and flow-rates in line with a standard electric geyser. One downside is storage; a solar-heated geyser loses heat considerably more rapidly than solar-charged batteries lose charge. The other is utility; no amount of hot water seems to ever charge my laptop.
Conclusion: Solar-thermal is an ideal, low-cost solution for low-temperature water-heating requirements. Solar-photovoltaic with battery storage is economical for electricity-only applications.
Diesel generator
It is an unwritten rule that NDAs apply only loosely when engineers are talking around a braai or over a beer. This honour code stipulates that names aren’t to be mentioned, only obliquely hinted at, and operational specifics are not repeated outside of that context. A result of this time-honoured tradition is that engineers all over South Africa know vaguely why the grid is such a mess, whose fault it is, and who gets rich from the situation.
Burning diesel for electricity is much more expensive than most other methods of generation. Eskom does it as an emergency contingency, purchasing the diesel from someone who gets very rich from the arrangement. We may eventually be surprised and shocked to find out who that is. In the meantime, you have the option of skipping out all that transmission/distribution infrastructure and burning diesel yourself.
Diesel engines are pretty inefficient at 25%–30%, but that’s not far behind our coal-fired power stations. In principle you could manage to beat Eskom’s overall thermal efficiency with a home diesel generator, albeit at a higher cost. However, generators have fairly narrow operating ranges for high efficiency and if your electricity requirements are not closely matched to the output of the generator then there are considerable losses. It is also a bit daft if you end up mainly using that energy for heat, because there are once again a bunch of inefficiencies as compared to skipping out the middle man and simply burning gas for heat, for example.
Conclusion: In practice, diesel generators for home use tend to be quite inefficient and dirty in terms of particulate emissions, as well as obnoxiously noisy. Overall thermal efficiency is around the same as Eskom, and all energy is drawn from non-grid sources, so these do alleviate grid demands.
