One that comes to mind that may work for your rig is pumping water to an elevated tank. To reduce bulk, instead of storing the water at the top, have an overflow return the water to the source "tank", so the pump never runs dry.
I agree.
This is a well-established method of storing electrical energy, used by power generating companies; called pumped storeage. They are a simple function of a power generator which can be operated in 2 modes: when generating, the pressure of water high up on a hill turns the turbines, generating useful electric power into the load of consumer's appliances. When storing, the 'excess' energy in the power grid being produced by other plants turns the turbines in the reverse direction, pumping water back up the hill. Thus the energy is 'stored' by the increased potential energy in the elevated water.
If the Wattage you've been producing is in the order of a load of car batteries then this will be fine - those operating in the UK are around 300MW, you can find them in Scotland and Wales.
And if that wasn't enough (!) you could just pump sea water!
Another form of storage used in some transport with re-generative breaking is to energise a flywheel, this works well for short bursts of energy input during breaking (which can then be fully released during accelleration). But even if the short-term nature of power excesses stored this was was acceptable, then perhaps the bearings for such a device would be beyond your abilities.
Alternatively, if you do find a way of generating useful energy, then why not just sell it to the generating companies in the same way that domestic owners of PV panels do? Use the load of other consumers' appliances, meter it and sell it. If the quantities produced are more modest, and a few carbatteries suggests that it is, then as Keith suggested, 20kW water heating elements are going to convert your electricity into measurable and recoverable heat quite simply. There's really no need to play around with corrosive battery acids or other dangerous materials and I would strongly advise against it.
I also advise simply dissipating the energy, use a commercial load-bank if you have to (you can hire them short-term from plant-hire companies).
Whatever you are trying to do, the merit in any 'new' design will be in its ability to generate useful and measurable electricity, not how you then store that energy, so concentrate on the important task of power generation. Clearly other people have better skills and experience than you in power storeage, so why waste time on making a poor job of the one part of the design which isn't original?