When I upgrade to 3 phase soon I will add electric elements to reduce the use of the gas burner. My main concern here is scorching the wort
The thing that governs scorching is heat density. The more surface area your elements have, the less scorching is an issue. Generally electric elements create a lot of convection in a kettle and don't cause scorching. That said, hot break proteins can get wrapped onto the elements and be a source of some scorching. Simply skimming any large chunks of hot break early in the boil will help.
My other issue is cooling 500 litres of hot wort. <edit> So if I cool from a cold water tank how do I calculate how much cold water I need at what temperature to cool the wort quickly & what is my best method? Physics was never my strongest subject at school (a long time ago)!
The basic formula is 3.966 btu is the energy to change 1 liter of water 1 degree Celsius. To save time, we can round that to 4 btu. If we calculate what you'll need for a lager, you should be set.
To chill to 10C for a lager:
100 to 10 C = 90 C
500 liter x 90 C x 4 = 180,000 btu
In most commercial setups, it's ideal to chill with the same amount of water you will need for a batch of beer. So, for demonstration's sake, let's assume that you need 700 liters returned to your hot liquor tank. Next, let's say you want the water to go in at 70 C
Now, it's a simple matter of reversing the above numbers. So for 700 liters:
180,000 btu / 700 = 257.14 btu/liter
257.14 / 4 = 64.3 C heat gained per liter.
70 - 64.3 = 5.7 C
Setting your cold liquor to 6 C should give you plenty of chilling power and the right amount of hot returned to brew with.