In your ZP/DP the VVVVV screen design being positioned next to the top covers would lose an excessive amount of heat at higher temperatures. The VVVV or finned design would be better served in the return pass.
It would be good to retain higher temperature air deep inside the collector when ever possible hence the reason for the double pass design. If the top screens were of a flat design 50% of the available light would be captured inside the collector. The addition of interior fins in the return path would seem to be a better arrangement (Cinefoil fins). An insulated interior return path (R12) for hot air is a must for a Double Pass system. My system uses screen modules with 1.2” spacing between screens and 2” VVVV Cinefoil separator between air paths. The air path ratio is 2 to1.
The collector I put together incorporated as many of the best practices I could find on the Simply Solar blog. I must say that I am bias toward this design because that’s I did it.
Here are some of the essential features that I think all Double pass systems should incorporate.
Finned double Pass collector research paper:
Use heaver duty galvanized perimeter frames to avoid screw pull-outs and allow brackets and handles to be safely attached anywhere. You will have more durable collector with a longer life span. 20 ga. Steel has a 104 pound pull out for a #8 screw.
No foam seals should be exposed to the collector interior. Use silicone based seals or cover seal with aluminum tape. Use lightweight aluminum sheet (aluminum soffit roll) for flex seals.
Use 110 VAC blowers with a variable speed control. 12 or 24 VDC fans are not good at pushing air.
Try to use the lowest noise fan when possible.
Use cushion fan supports and/or cover it up whenever possible.
Use a programable temperature control for primary fan control, not a snap switch (use one as a backup alarm). You will be able to control the temp levels and on/off differential much better.
Inlet filters will reduce air flow by 20%. If used, double or triple the filter size unless you want to purposefully reduce your system air flow.
Tilted south facing collectors are 15 to 20% more efficient than vertical ones. If a vertical collector is used, a fold down reflector cover can double as summer season coverup.
System cost – US$ -- Two 4’ x 9’ collectors
I have lot build pictures of the VVVV design but have not published yet.
Jim from IL