Now for a slightly larger project: Designing a medium-sized commercial solar thermal system

Solar Phoenix drainback system with additional storageThe last blog post discussed light commercial systems for restaurants and other similar businesses. But what if you need a solar thermal system with a bit more uumph? Here’s the recipe for a solar hot water system that would be perfect for, say, apartment buildings or hotels. It provides 330 gallons of hot water per hour, with significant backup strength.

The example I’ll discuss below is for a two-building apartment complex in Los Angeles, in which the solar thermal is targeted to provide 50 percent of hot water demand.

This size of solar thermal system –six solar collectors backed up by a Solar Phoenix modulating water heater — also works great for large restaurants or smaller hotels. To illustrate this, take a look at this case study of a solar thermal system on Catawba College in Salisbury, NC (click here for the case study). Cost analysis indicates that energy costs at the school were cut by 58 percent. Now that’s a success story.

Now, here are the details on the new, low-income housing project in LA that needs a cost-effective solar thermal system:

  • Building A has a hot water demand of about 500 gpd. It’s made up of 16 two- and three-bedroom units; a laundry room with four washers; a community kitchen; and a staff restroom.
  • Building B has a hot water demand of 800 gpd. It includes 25 one-bedroom units; three washing machines; and one mop sink.

Solar thermal system at Catawba College; see story for link to case study

The solar thermal system I designed for this apartment complex features a 199-BTU Solar Phoenix, a highly efficient water heater that stores nearly 120 gallons of solar-heated water and kicks in as backup with more water in needed than can be heated by the sun. It’s a drainback system to guard against overheating. Also, I’ve upped the solar water storage with the addition of a second 119-gallon tank. While a light commercial system may require three to four solar collectors, for this application, I’ve recommended six 4’x’8’ collectors.

Here’s the parts list:

  • 1 ea.  Solar Phoenix, PH199-119SNHX, stainless-steel, modulating, condensing water heater with solar input
  • 1 ea. SSU-119 stainless-steel 119-gallon indirect tank
  • 1 ea. SSU-20DB stainless-steel drainback tank for overheat protection
  • 1 ea. Variable speed solar pump control with 4 sensors (8600-047)
  • 1 ea. Solar-rated anti-scalding valve (8600-068)
  • 6 ea. 4’x8’ flat plate solar collectors (FP-32SC)
  • 6 ea. FP-RM mounts (or select mounts for roof application)
  • 1 ea. Field-supplied pump that will supply 7 GPM and the lift from the drainback tank to the collectors
  • 1 ea. Field-supplied tank-to-tank transfer pump 1/2″ (I recommend a Taco 006B)
  • 1″ copper and insulation to the collectors

The control strategy uses a tank-to-tank transfer pump that will transfer the solar heat to the water heater, but will not allow the water heater to heat the storage tank. This is not a solar tank preheating a water heater; rather this is solar heating 240 gallons of storage and the water heater only firing to make up what the solar will not produce.

Data logging that allows solar production to be posted on the internet is available if the apartment building supervisors have an interest in seeing how well the solar system is performing.

In colder climates, I would recommend increasing the number of collectors to eight collectors or upgrading the size of the collectors to 4’x10’ panels.

For more systems featuring the Solar Phoenix, see the Solar Library on this site.

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