I would have thought that you would want to conserve as much energy as possible - or is energy/electricity not a problem?
Yes, you would like to conserve the energy, but sadly that is not possible. The problem is: The energy you have is heat.
The second law of thermodynamics states that entropy always increases, and that means that while every process in the spacecraft produces heat, going in the opposite way is impossible. You can produce energy from heat if you have a temperature gradient, but if you have that on board, you would only be moving the heat around in any case.
In short, everything going on produces heat, and you can not magically transfer that back to useful energy. The only solution is to dump it.
Although there is technology which can convert waste heat to electricity you cannot do it economically.
Energy is precious in a spacecraft, the more you need the bigger solar panels you need to generate it, the bigger batteries you need to store it, or the bigger nuclear thermal generator you need. Bigger means heavier, and heavier means more expensive to launch. When electricity is converted to motion, or used in computers/sensors waste heat is generated, the amount of energy lost can be limited by using more efficient materials. There's a limitation to how efficient you can get, so you will always have waste heat.
Keep in mind that you want some heat generation in most spacecraft so it doesn't get too cold to operate. Waste heat is one way, some spacecraft have heaters to convert electricity into heat when in shadow. So waste heat is actually useful.
There are devices that can convert heat into electricity, called thermo-electric generators. These are used in spacecraft already in nuclear thermal generators, aka radioisotope thermoelectric generators which are used in spacecraft going to places where there isn't enough sunlight for solar panels. As radioisotopes break down they produce heat which is converted into electricity to power the spacecraft.
Thermoelectric generators could in theory be used to convert waste heat back into electricity, but you'd lose more than you'd gain as the weight you'd gain from them would not be offset by less solar panels and batteries - they are heavy and inefficient, with 5-8% typical energy conversion. This means you'd still have 92% of the heat to radiate. Newer materials are being developed that may bring this to around 20% efficiency but that would still not be enough to make them worthwhile.