Solar hot water. One or two tanks.
mikemahan
| Posted in Energy, Heating & Insulation on
My solar hot water system has been down for a couple of years, due to a frozen collector and unrelated roof problems. The system is active, with electric back-up. I’ve used two tanks in the past; one for solar storage that feeds solar hot water into an ordinary electric water heater. It looks like I need to replace both tanks. Is there really any advantage to two tanks?
Replies
I am not a solar expert but here's what I understand. You might benefit from a drainback system using two tanks. One tank (small) circulates water thru the panel when the system runs. When the system is not running the water drains back to the small tank. A heat exchanger inside the small tank is connected to a larger storage tank. Since you previously had freeze damage I would consider something like this. We have installed one solar thermal system, pretty much as described above--small drainback tank, 80 gallon solar storage tank, 50 gallon electric water heater. Water coming into the house is heated in the 80 gallon tank before being drawn to the 50 gallon tank. The idea is to keep the 50 gallon tank from running. Owner has learned things like doing laundry in the afternoon, etc.
we've always use a two tank drain-back system
you usually have a higher delta-T if you use a solar storage tank
than if you use your DHW tank
your DHW tank wants to be say 105 deg to say 140 deg
but your solar tank can be whatever it is.. so say it starts at 80 and during the day you get it up to 160
if you had been using your DHW tank, your system probably wouldn't have done any collection for half the day
most soalr controls use a "turn on pump " at delta - t of say 10 deg and off at 5 degMike Hussein Smith Rhode Island : Design / Build / Repair / Restore
I tend to agree w/ Mike Smith. Two tanks gives you flexibility and extra capacity for storing excess energy. That COULD be done w/ one, but there may be some logistical issues I can't think of right off the top of my head. An extra tank can be cheap ... I got a used water heater for $0. The heater doesn't need to work. Lots of these available ... just talk to your local plumbing supplier ... he should have one in his bone yard.
The second tank (place before your conventional tank) acts as a preheater ... so if your storage is only 90 deg ... the conventional heater works a lot less than the say 55 degF water coming in. The energy aspect isn't much different. Hot water is hot water ... takes 'x' amount of energy to heat it up for use. Two tanks have more standby losses ... but properly insulated, this is a small issue.
If you have freezing issues ... you should have 1) a glycol based system w/ heat exchanger 2) a drain back system that drains into a small holding tank or 3) a drain down system that drains to the sewer (it's a small amount of water). Your system needs to respond to a freezing condition reliably. But that shouldn't be too hard. Controls are the key, though, but this is simple controls, relatively speaking (although older systems seemed to be plagued by control issues for some odd reason).
Why do you have to replace both tanks? Are you without hot water right now? Does the preheat (i.e. solar) tank actually leak?