It's more like several ideas, based on same idea. And idea is that heat is a measure of energy.
Heat is the only energy form in vanilla (furnaces). Most, if not all, mods that have any kind of energy, has means to convert heat into energy or use energy to make heat.
Also, there is always a confusion about rate of conversion between different types of energy, and heat can be universal "currency" of energy.
"Heat battery" is an item that holds heat. Simplest form is a stackable "small" battery that can hold tiny amount of heat - namely, heat, equal to heat generated by furnace in 10 seconds (200MJ, 500EU, one smelted item). It's 32k per stack.
It has no charge level, just two forms - empty and charged. So, just by adding furnace recipe, it can be made chargeable in all furnace-compatible devices - vanilla furnace, IC2 electric and iron furnace, BC APS furnace, and even in RP2 blutricity furnace.
Such batteries can be unloaded in geothermal generator (may be at less-than-100% effectivity). Geotherms should have extra slots for empty batteries (can be also used for empty (former lava) buckets)
Recipe can be - redstone at top, tin in corners, water (bucket or cell) in center, iron on sides and bottom. Giving several - 16 or so - small batteries.
Second battery type - "big" battery. It is not stackable, but can be charged gradually, up to equivalent of 60k EU heat (or 24k MJ, or 20 minutes of furnace working).
Recipe is the same as for "small" batteries, but with Lapis instead of Redstone (so, 4 tin, 3 iron, one water and one lapis), and gives only one battery, instead of 16.
First, it can be charged in furnaces. Not sure it's easy to make it chargeable in vanilla furnace, though.
And it can be unloaded in geothermal generators, naturally.
But it's main function is to work in reactors. It does following.
It absorbs all heat dropped into it, like condensators do. It adds this heat at a certain ratio (1eu per 2 heat or so) to it's own heat charge. And it reduces reactor eu/t output by same amount. So, several of those used in reactor can easily drop output to zero. Justification for it is that while other cooling elements only consume part of heat, passing most of it to reactor's integrated generators, Heat Battery consumes it all. Then that cells are supposed to be taken out and cooled/uncharged in geothermals.
Effective EU/t for such setup can be up to 37 eu/t per uranium cell (compared to 42 eu/t for 1.2.5 CASUC), but instead of 4-8 bucket deployers, it require huge amount of geothermals - around 150 for tightly packed reactor. As well as logistic system for rotating cells.
Compared to condensators, it does not require autocrafting table (so, BC (or CC) is not mandatory), cobblestone generator and mass fab. It is more expensive (in iron, especially), but gives more energy per uranium (can be changed by altering reactor heat-to-eu ratio). And, yes, it feels more "realistic" and "industrial" to operate with heated water and generators, instead of magic manipulations with lapis. For me, at least, it is a plus.
