The amount of solar energy that reaches Earth’s surface is enormous and is naturally one of the main reasons why this is a habitable place for humanity. Because the planet is round the surface receives the solar rays at different angles at different locations. As the Earth’s axis towards the sun is tilted by 23.5°, places further away from the equator receive more solar energy in some parts of the year than others. At the latitude of 40° (like for example in Madrid) the surface receives three times as much radiation in June than in December.
The lower the angle, the further the radiation has to travel through the atmosphere, and the less of it is left when it reaches the surface. So at the poles the angles are so low that very little energy gets through, and in wintertime there are long periods without light.
The suns elliptical course around the sun during year (it is closer to sun when it is summer in the southern hemisphere) does not amount to enough difference for making the southern hemisphere hotter than the northern during their respective summer periods, as this difference is mostly absorbed by the oceans.
When we use fossil fuel we are really using the energy from the sun that reached us millions of years ago. That energy was first stored in organic form, like for example plants, by a process of photosynthesis and then decomposed deep below the surface into energy forms like coal, oil or gas. That process is still going on, but is so slow and our outtake of fossil fuels so vast that it can’t be considered a renewable energy source.
The problem with not having the same amount of solar energy all the time is that you would want to store the received energy on a sunny summer day and keep it for a cold winter night, which is of course much easier said than done. First you would need a very big storage capacity (which is expensive) to able to retain enough energy for when you need it and can’t get it and, secondly that storage should be absolutely stable so that what you captured is not slowly leaking away.
A very interesting on-going research project at the University of Chalmers in Sweden is trying to use chemical solar capture with the help of an artificial molecule (made up of carbon, hydrogen and nitrogen) as a way of creating a durable energy storage. This molecule has different forms (isomers) with different energy levels and when it is exposed to sunlight in a solar collector it changes from a low-energy form to a higher energy form, storing the energy inside its chemical bindings.
The molecule can then be stored for up to 18 years without loosing it’s new form and when the energy is needed the liquid containing the molecules is passed through a catalyser, where heat is produced and the molecules turn back to their original form. The good thing is that the system works as a closed circuit as the molecular liquid can be passed through the solar collector again and again to store new energy.