laptops typically require 19 V DC for charging while commercially available peizoelectric sensors generate 10 mV to 100 mV DC per strike. practically voltage output from a peizoelectric sensor can be enough to power an LED. your project concept is amazing. you first need to make an estimate that how much voltage will be generated at the capacitor by an array of sensors placed under keyboard in a short span of time so that on average rate of typing on typical keyboard (which has 107 or 105 keys), what average DC voltage is obtained. That voltage will be needed to be amplified to make a charger circuit for the laptop battery. here 19V is only enogh for 3 and 4 cell laptop batteries. 6 cell or 8 cell laptop battery will require even more DC voltage ffor charging.