1. Solar
a) Direct heat
b) Thermal electric generation
c) Photovoltaic electricity
d) Solar derivatives such as OTEC (Ocean Thermal Energy Conversion), wind energy, waves (a wind derivative), hydroelectric

2. Biomass
a) Biogas from hitherto "waste" derivatives of farming.
b) Non-destructive vegetation products
c) Firewood
d. Plants grown for energy use

3 Other income sources

a) Tidal
b) geothermal

4 Nuclear fusion see Solar (as there are no signs that this technology will be available in any foreseeable future.)

All the above sources of energy need to be assessed for their effects on the world system as a whole. For example, there may be some negative features to some "biomass" sources, which may not be useful to the system as a whole. The most obviously beneficial are the various derivatives of solar input. The most obviously dubious are some biomass sources that turn food into motor fuel or rain forest into palm oil.
Thinking about these energy sources must be much more comprehensive than thinking about oil and gas.

1. Solar

• Direct heat
  There are many processes that need heated water or air. Where there are frequent cloudless skies direct heat can replace many uses of "conventional" energy. An obvious use is domestic hot water. Even in such areas as Britain solar water heat can save large amounts of gas or electricity. Generally, it is effective on sunny days for six months of the year. In areas nearer the equator it is even more effective. (The writer has experienced solar heated water as far north as Stockholm, where presumably the effective season will be shorter than in Britain, but still not negligible).

  There may well be industrial processes that can use direct solar heat.

  An objection to using direct heat is that it is subject to weather conditions. Thus, if processes must happen whatever the weather there must be back-up systems. But sometimes a change of behaviour can be useful. For example, as the owner of a solar water heating system I wash the clothes on sunny days rather than cloudy and use gas only when necessary. Income sources of energy in general may require more flexibility in our behaviour.

• b) Thermal electric generation
  Cloudless arid areas (deserts) may be useful sites for thermal electric generation - mirrors concentrating sun energy on to a heat engine. Like the generators reliant on coal these tend to be large and centralised, feeding power into a grid. In southern Europe and North Africa there may be many sites useful for this mode. Here is a useful article.

  A feature of this method is that power changes with time of day from zero at night to a maximum at solar noon. A grid reliant on this mode must have other sources, or must control the consumers with automatic cut-offs (a technology that interrupts supply when generation is limited, or demand is high).

  As with direct solar heat our behaviour needs to adjust to the energy source.

• c) Photovoltaic electricity (PV)
  At present photovoltaic electricity is in wide use for small uses that could not be supplied by other energy sources. For example rail and road monitoring systems use PV panels to power instruments and transmitters. As long ago as 1985 I was told by a rancher in Australia that it provided the best method for powering remote water pumps on his vast sheep station.

  Already many individuals and institutions use solar electricity based on the roofs of individual houses. Clearly urban roofs provide a huge source of space where PV can be based, to be used mainly on site.

  PV of this kind has two main modes of use:

  1. Stand alone
  For houses sited off the grid PV electricity can be made during daylight and stored for use at night usually in the form of storage batteries. As these commonly use lead and acid there are probably limits to the number of this kind of installation. Lead is a limited resource which surely could not support an unlimited expansion of the use of this type of battery. Other types of battery are at present more expensive.

  2. Grid connected
  If the PV on roofs is connected to the grid the user's meter can be made to run backwards when the building is making more electricity that is being used, and forwards when more is being used than generated.

  Here we need to consider the effect on the grid as a whole. One often cited negative feature is that for night time use other generating capacity would be needed on standby. However, as with solar heat, there is the possibility of behaviour change - that certain uses are turned off when there is a lack of sunlight.

Storage
Research and investment may be needed on storage techniques. For example in the British system there is already a pumped storage system (near Blaenau Ffestiniog) that pumps water to a high reservoir and releases it when extra demand occurs. There may well be scope for more of this kind of storage.

Necessity should produce other means of storage, such as hydrogen and its derivatives (methanol, perhaps), for storing energy produced in the Sahara.

d) Solar derivatives

• OTEC (Ocean Thermal Energy Conversion),

  A derivative of solar energy because it depends on the temperature difference in tropical areas between deep cold water in the oceans, coming from the ice caps, and surface water heated by the sun. An advantage is that it can supply power night and day. Is it dangerous for the warming of the deep water and release of methane hydrates? Probably not if the amount is limited carefully.

• wind energy,
• waves (a wind derivative)
• hydroelectricity

2. Biomass
Is biomass the answer to all our problems? Probably not. However, even though it has a role to play, the attempt to replace fuel for private cars with biomass alone would certainly ruin the planet.

• a) Biogas from farm wastes see Biogas Index
  There is a huge amount of material that could be turned into biogas for local use. One problem with this technology is that it is unlikely to produce energy that could be sold to the industrial world. Instead it could transform the lives of many people who at present live outside the modern fuel economy, replacing not so much oil products as firewood. Biogas can be made most easily and cheaply in tropical countries, but it also has a role in cooler areas - such as the train running on biogas from a slaughterhouse in Sweden.

  See for example the Hyacinth Project. Water hyacinth is at present a pest but could be used as a solar collector, digested with animal wastes, and produce numerous useful substances, none of which add to the climate problem.

• b) Alcohols from carbohydrates - such as sugar, maize, wheat etc.
  There are real doubts about how much these can contribute. Growing food and then fermenting the carbohydrates needs many inputs, at present supplied from oil products. For example, fertiliser is an oil product; distillation requires an energy input. If this comes from the conventional power system, how much extra, solar-derived, energy will the whole process actually deliver? This system looks attractive to politicians anxious to make their voters believe they can continue to use their cars as much as they like (and as a hidden subsidy to rural voters) but may have no real role in an income energy system. If it takes land at present used for growing food a side effect could be famine (as has occurred in 2008).
• c) Firewood
  When the human population was smaller firewood supplied most of the cooking needs, and did not damage the forests because it was not taken faster than could be replaced. Present day populations press on the ability of forests to replace what is taken, especially when the wood is transformed into charcoal - a very wasteful process.
• d) Plants grown for energy use
  There are possibilities of plants with hydrocarbon sap that can be grown in semi-arid areas. However, there are not yet any working systems so it is not possible to say whether these will provide a genuine contribution to human energy needs nor whether there would be any bad effects of doing so.

3. Other Income sources

• Tidal movements depend on the gravity effects of the attraction between the earth and the moon. Certain areas (such as the Bay of Fundy and the Severn Estuary) could produce huge amounts of energy from the rise and fall of the tides. Other areas of ocean have tidal streams which can be harvested by stationary wheels.
• Geothermal heat derives from hot rocks, sometimes producing steam. There is probably scope for more than is developed at present. Iceland uses more than any other country but New Zealand and Kenya are also important producers.

The end of Piracy?
The difference between the income sources of energy and the present fossil fuel economy is like the difference between the Spanish Empire and its methods compared to a modern economy. The conquistadors saw the new worlds they discovered as if they were a pirate band - somewhere to be plundered. They looked for gold wherever they went and when they found it, stole it. Nothing was made. When they got the gold home it caused inflation and a huge rise in imports. The economy of making things faded away, leaving Spain impoverished and bankrupt.

The oil and coal economy is similar. These energy sources began to be plundered in the 18th century. We should see those great companies such as Exxon, Shell and BP as pirate bands. They look for the energy supplies in the earth, and extract them. It's easy money, compared to actually making things. When they are gone, they are gone. Even if this process didn't threaten catastrophic climate change it's a daft way to behave, as when they are used up we would all be like Spain after Empire - bankrupt.

By contrast the quantity of income energy that can be harvested depends only on the amount to be invested in collecting it. Once the apparatus is in place there is only the cost of maintenance and depreciation.

Are the oil and coal companies the best organisations to manage these industries? Probably not, as they have large parasitical bureaucracies, paid for by their huge profits. Income energy is unlikely to support large numbers of unproductive managers. It is more like farming.

It is certainly possible to have a civilisation that has machines making use of the sources of energy listed here. It may not be possible to have some of the things we have now, which are powered by the temporarily available fossil sources, at the expense of climate change.