Nuts And Bolts:
The energy in a square foot of sunlight:
The solar energy striking each square foot of Earth each year is roughly equivalent to the energy in 5 gallons of fuel. That is 5 gallons of energy, every year, on every square foot, and that's a LOT of energy. The trick is to harness it economically. Most photovoltaic systems collect about 15% to 20% of that solar energy under optimal conditions. Solar thermal collectors can collect about 4 times that percentage, or up to about 70% of the Sun's energy striking them in optimal conditions. So using solar thermal collectors to meet daily hot water needs is one of the more efficient ways to harness solar energy.
Major Elements:
Solar hot water systems require
Collectors (which collect the free heat from the sun)
Storage (one or more tanks to store the heat until needed)
System piping & controls.
 
Bulk Drainback System:
The system we favor for large domestic hot water applications is the 'bulk drainback' system. Some benefits are: 
Failsafe freeze protection. The piping and collectors are pitched so that when the pump for the collector loop shuts off for any reason (including black-outs), the water in the collectors and piping all drains back safe and snug into the insulated storage tank, so there are no worries about freezing collectors. Unlike 'drain-down', or 'recirculation' systems, 'drainback' systems are not dependent on electricity for freeze protection.
No heat exchange chemicals are used. Unlike glycol systems, our drainback systems run on plain water that does not turn acidic; our drain back systems do not need periodic fluid changes; and our drain back systems allow for extra long collector life.
The thermal storage tanks in bulk drainback systems have no need to be pressurized. On large systems, this turns out to be a big plus, allowing more storage for less cost.
 
Make Hay When The Sun Shines, Then Store It:
Storage capacity can dramatically affect overall system performance. The rule of thumb for solar hot water storage is to store at least one day's worth of solar pre-heated water. In theory, as hot water is drawn from the existing hot water heater, it is replaced by solar preheated water that is stored in the solar tank. By the time one-day supply of solar preheated water has been fed into the conventional heater, the sun in theory is already heating up the next day's worth of solar hot water. The 'one day' formula works well in areas where the Sun shines fairly reliably each day, such as in the Southwestern Sunbelt. In order to get high performance in areas such as Philadelphia, with substantial cloudy periods, we have found we get excellent results incorporating 2 day's or more of storage. For example, this could translate to about 1,500 gallons of storage for a 20-unit apartment building.
 
Big Tanks / Small Doorways:
Many existing buildings have space in the basement or boiler room for a large tank, but many of these same buildings lack large doorways to bring in oversized tank(s). The non-pressurized STSS tanks we use (www.stssco.com) can flatten out to pass through standard doorways. In the case of very large tanks, we are able to bring in the tank materials (the aluminum structure, the insulation, and the long life EPDM liner) in manageable size components, and assemble a high performance long life tank on location. The ability to site assemble these large tanks is a key element to being able to provide economical solar hot water systems for existing large buildings.
 
Collectors:
The technology of solar thermal collectors has been well proven over multiple generations. The photo at right shows a system in the early 1900's.
 
We have used different brands of collectors, both flat plate and evacuated tube. For reliable long life and for high performance in typical domestic hot water preheating applications in our area, we favor quality flat plate collectors, with selective surface copper absorbers, low iron tempered glass glazing, premium insulation, and extruded aluminum frames, such as the www.SolarH2ot.com platinum series shown in our installation at right. The photo at right shows a 15 panel installation of ours on a 22 unit apartment building.
 
Racks:
Here is where our craftsmanship shines:
Our racks are engineered to withstand more than the maximum recorded wind speeds in the area with strength to spare.
Our structural members and fasteners are maintenance free aluminum, stainless steel, or hot dipped zinc.
On flat or nearly flat roofs, we aim to have all roof mounts not only well sealed, but also raised above the surrounding roof so as to be above any potential standing water.
Our racks are installed with future roof maintenance in mind. On built up roofs, we leave room for maintenance personnel to be able to periodically recoat the roof under the racks.
When required, we can get professional engineer certifications for custom racks.
 
Space requirements:
Below we try to give a rough idea of what space will likely be required for
collectors and tanks when installing an effective solar hot water heating system:
Alignment: In general terms, it is best if the long side of the building runs East - West to allow for a long array facing South. In and ideal world, the solar thermal collectors would be aligned to face about 5 degrees West of South. Aligning the collectors slightly West or South (as opposed to due South) allows some early morning sun on the collector, to be traded for some late afternoon sun on the collector. Since it is generally warmer in the afternoon, this allows the collectors to run at a slightly higher efficiency over the course of the day. Having said that, anywhere within 15 or 20 degrees of ideal is still quite acceptable. 
Length of array: For apartments, if one is aiming to reduce hot water heating costs by 50% or more, we have found about 1.5 lineal ft of 8' high collector per tenant is required. In other words, on a 30 unit building with 40 tenants, the roof would ideally have enough sunny open space to allow installation of an array with a total length of about 60 feet. 
Tilt of the array: For applications with fairly steady year round hot water requirements in the Philadelphia area, the collectors would ideally be tilted up at about 55 degrees from horizontal. To mount flat on existing sloped roofs, one can stray several degrees from the ideal pitch and still get good performance.
Tank space: Adequate solar hot water storage is key to achieving a high percentage
of savings, particularly in areas like the Philadelphia area that experience significant cloudy periods. For apartments, we get best energy saving results when we have 50 gallons or more storage per occupant. For example, on a 30 unit building with 40 occupants, this could equate to a 2,000 or more gallon tank that might have a footprint of about 8' to 9' diameter and a height of about 6' or 7'. This tank would typically be located in a basement or on a first floor slab-on-grade. We will be happy to come out and assess if your building is a good candidate. 
  
Efficiency:
Our electric swing hoist allows us to safely and easily bring collectors and other materials up to the roof. We purchase wide heavy gauge aluminum coil stock in bulk. With this material we are able to built extra large tanks simply and efficiently.