Has an all aluminium absorber the same performance as an absorber with copper tubes?
Hydro: An all aluminium absorber will have the same performance as one with copper absorber tubes. Please see next question and answer.

Aluminium has a lower heat transfer coefficient than copper. Will this result in a lower performance for an all aluminium absorber?
Hydro: This has no practical influence on the absorber performance as the heat transfer from tube to liquid is the bottle-neck.

Aluminium has a higher thermal elongation coefficient than copper. Will this cause higher thermal stresses in the absorber?
Hydro: No. Having both absorber plate and tube in the same material will both reduce stresses in the welding and the risk of out-of-plane deflections of the absorber.

Aluminium is not as strong as copper, will the absorber work alright with today's temperature and pressure loads?
Hydro: Yes. Aluminium tubes may be softer than copper tubes, but they withstand a lot more pressure than their reputation gives them credit for. The dimensions offered for solar thermal systems can typically hold 50 - 100 bars burst pressure, depending on dimension. This is significantly more pressure than present in any solar thermal systems, and offer plenty of safety against burst.

Read more about heat transfer in aluminium absorbers
Read more about absorber warping