I guess natural brooding is obvious but it does work... About 2 years ago I let one of our hens incubate and brood her own chicks - it was about 10 all up and they could hardly all fit under her bum.
Brooding is easier than incubating because the temperature can fluctuate a little without too much dramas and as long as the chicks can get to a warm sheltered spot they should be fine. In summer (particularly warm climates) there isn't really a need for a heat source at all.
The problem with natural brooding, where the chicks are either isolated in a warm area or left with the mother, is the numbers because there's only so many chicks that can be raised this way. Artificial brooding with a light, heat lamp, or other heat source is more productive and controlled.
Are you planning to incubate with power and then brood without or both incubate and brood without power? I can't imagine how it would be possible to make an effective incubator which doesn't use power - how would the temp be regulated accurately? I'm thinking if you wanted to raise poultry small scale commercial you'd really need a powered brooder; however, if you wanted to breed for your own meat only then you could encourage natural incubation and brooding, but others might have some better suggestions.