Hi,
6 l/sec = 12.713 cubic foot/minute
A good site for doing conversions is: http://www.onlineconversion.com
So, your heat output is:
Qout = (air weight flow)(Tout - Tin) (Cair)
Qout = (12.71 ft^3/min)(0.065 lb/ft^3)(150F - 37F)(0.24 BTU/lb-F) = 22.4 BTU/hr or 1344 BTU/hour.
Don't know how large your collector is, but lets say its 20 sqft (you can adjust the calcs to the actual area of your collector).
Solar In = (300 BTU/sqft - hr)(20 sqft) = 6000 BTU/hr
Efficiency = (Heat out)(Solar in) = (1344 BTU/hr)(6000 BTU/hr) = 0.224 (22 %)
The 300 BTU/sqft -hr is for full sun, and for the collector being pointed right at the sun. Its best to do this test around solar noon, and aim the collector right at the sun.
The temperature rise of 150 - 37 = 113F is a lot -- the collector could use more airflow for better efficiency.
How large is the collector?
Gary