I think you forgot the demand in the sum. The lower bound for storage is too low in your cuts. Whether 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 them.) 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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