Yet another Hacker News

27 points 0sake_rs 1 hour ago 12 comments

jpm_sd 44 minutes ago | parent

title should be "Sustainable Energy – without the hot air"

It's a bit out of date at this point. The author passed away in 2016.

https://en.wikipedia.org/wiki/David_J._C._MacKay

_aavaa_ 38 minutes ago | parent

The books is good(ish) for it's time, but some of its analysis and forecasts are fundamentally flawed since it falls for the primary energy fallacy by comparing the chemical potential energy (in J) directly to electrical energy (also in J). The two are fundamentally different things and called be compared 1:1. E.g. To heat up your home with natural gas you need ~1J of chemical energy to get 1J of heat into the home, but with a electric heat pump you only need 1/6 J to get 1J of heat.

It is also a product of its time in terms of wind/solar vs nuclear. His forecasts of the impact of solar and wind is based on prices and performance from 2008. Prices have come down an order of magnitude since then, and performance and lifespan have increased drastically.

azornathogron 24 minutes ago | parent

Heat pumps, and their efficiency (getting more than X Joules of heating for X Joules of electricity) are discussed in chapter 21 Smarter Heating, see for example the diagram and discussion on page 150: https://www.withouthotair.com/c21/page_150.shtml

I imagine the economics (and perhaps technical improvements) of some things may have changed more than the book could forecast. But heat pumps were definitely understood by the author.

bryanlarsen 17 minutes ago | parent

Which means that not including the conversion in the primary comparison is particularly egregious.

_aavaa_ 9 minutes ago | parent

I’m not saying he was unaware of heat pumps, I’m saying he’s comparing apples to oranges simply because they use the same units.

Look at “3 - Cars” on page 29. He says the typical car uses 40 kWh/day. 40 kWh of what? Chemical energy in the gasoline.

The go to page 33 where he looks at how much energy onshore wind could produce per days in the UK. His number is 20 kWh/d. 20 kWh of what? electricity

He then compares those two numbers directly and uses that comparison as the basis of his arguments: “Britain’s onshore wind energy resource may be “huge,” but it’s evi- dently not as huge as our huge consumption.”

This is simply incorrect. A combustion engine converts less than half of the chemical energy in the gasoline into mechanical work that can move the car. The electric model converts >90% of it. So we don’t have to replace 40 kWh/day, we have to replace less than half of that since the electric process is more efficient.

This same issues, the primary energy fallacy, underpins large parts of the book.

bryanlarsen 18 minutes ago | parent

> Prices have come down an order of magnitude since then

$10/W to 0.30/W is closer to 2 orders of magnitude than 1.

perennialreport 23 minutes ago | parent

Worth reading even with the dated numbers. MacKay's lasting contribution isn't any specific forecast - it's the discipline of expressing everything in the same units (kWh per day per person), which makes energy arguments commensurable instead of vibes-based. Once you've internalized that, most energy coverage in the media starts reading like category errors. The specific figures have moved - solar costs, EV adoption, heat pump uptake - but the arithmetic habit hasn't aged at all.

sam1r 22 minutes ago | parent

The original author was blogging just a couple days prior to his death. Rest in peace

https://itila.blogspot.com/2015/09/what-do-you-tell-children...

peri-cl 13 minutes ago | parent

The author is coincidentally the author of a (free-to-read) information theory book which is also popular on HN,

https://news.ycombinator.com/item?id=34618613 ("Information Theory, Inference, and Learning Algorithms (2003) (inference.org.uk)")

esafak 12 minutes ago | parent

The late author.

js8 11 minutes ago | parent

Love the book, but as others have noted, it is very much dated. Although I believe someone started a project some years ago to update it.