A cornucopian is someone who posits that there are few intractable natural limits to growth and believes the world can provide a practically limitless abundance of natural resources. The term 'cornucopian' is sometimes used derogatorily, especially by those who are skeptical of the view that technology can solve, or overcome, the problem of an exponentially-increasing human population living off a finite base of natural resources.
In practice, the cornucupian view relies upon the economic law of supply and demand: as demand rises for any commodity, its price rises accordingly, inducing producers to deliver more of it (since they stand to earn more per each unit sold). The subsequent increase in production tends to lower the price and make the commodity available to a wider range of consumers. This view assumes that the resource in question is renewable, or so vast that continued exploitation of supply can continue to meet demand.
Joe is a cornucopian because he believes the world will be saved from oil depletion by Solar electricity.