I like solar cooking ideas, and this one is great because you can cook anything on it that you would cook on a stove top. Solar cooking is a great alternative, especially during the hot summer month when the last thing you want to do is turn the stove on inside the house.
You can buy the fresnel lenses on ebay, and the are great for a variety of different solar projects. Remember if you purchase a fresnel lens, you need to wear eye protection, and be very careful where you point it. Good luck with your solar cooking.