I think, you forgot the demand in the sum. The lower bound for storage is too low in your cuts. If fixing k and l helps you depends on your problem and how often you produce in the optimal solution (If you produce in (nearly) every period, it is better to fix it). I think Wolsey offers the simple option l=k as a good choice. I have the same problem ( capacitated ) and it is very difficult to find good cuts.
sprtka
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