Exporting interpolating functions to excel

The interpolating functions that NDSolve returns contain an irregular grid that reflects which points were used to calculate the solution. Not always, but often this grid is a better choice than a regular grid as you would generate with Table when exporting. Here is how you could export the data as NDSolve generated it:

a = 10^-2;
eq1 = {hf'[t] == -a*(hf[t] - hs[t]), hs'[t] == a*(hf[t] - hs[t]), 
   hf[0] == 20, hs[0] == 0};
sol1 = NDSolve[eq1, {hf, hs}, {t, 0, 100}]

hfsol = hf /. First[sol1]
hssol = hs /. First[sol1]

data = {#, hfsol[#], hssol[#]} & /@ First[hfsol@"Coordinates"]
Export[FileNameJoin[{$UserDocumentsDirectory, "sol.xlsx"}], data]

a = 10^-2;
eq1 = {hf'[t] == -a*(hf[t] - hs[t]), hs'[t] == a*(hf[t] - hs[t]),  hf[0] == 20, hs[0] == 0};
sol1 = NDSolve[eq1, {hf, hs}, {t, 0, 100}]

Now:

Plot[{hf[t], hs[t]} /. sol1, {t, 0, 100}]

Export["c:\\test.xls", Table[Flatten[{t, hf[t], hs[t]} /. sol1], {t, 0, 100}]]