Go to a coin car wash with a bottle of simple green. Spray simple green all over engine bay, use spot free rinse to rinse clean. You can use your hose at home but you may get some water spots.
Done.
The alternator will be fine if you get a little water on it. Just use common sence when rinsing. Don't spray directly at sensitive components with full rinse pressure. I light rinse will get the simple green and dirt all off.