Greenpowerscience sells Stirling Engines

I've posted on these that Dan Rojas has worked on.

The question is this:  can this do useful work, and how much?

The link to the sales page.

It isn't out of the question that such can be built.   Another question should be is why aren't they being built?  Why only models that can't do anything useful?

What I'd like to do is simple: Use one of these Stirling Engines to help recycle water and carbon dioxide.  It does this by the pyrolysis of algae, which is quite prolific.  The gases produced will be combustible, which can be used to run one of these Engines. It would produce electricity, water, and carbon dioxide.  The water and carbon dioxide are recycled back into the production of more algae.

Update:

The algae can clean up the water too, as a high pH water resulting from washing and cleaning is good for algal growth.  This can be the first step in reclaiming the water.  The other steps are the ones I've been working on in the water filtration lab.