Question: What is the meaning of Delta and for calculating the power of a design?

Answer: You can regard Delta as the smallest change in response that you want to be able to detect in the experiment, and is the expected noise level. Together these define the signal-to-noise ratio, which is used in the calculation of power. If the signal-to-noise is low, you will need many measurements to achieve an acceptable power (or probability of detecting small, but real effects).