612 m2 flat plate collector
Prinshof student residences, University of Pretoria.
Holms and Friends: 45,800-Litre high pressure mild steel vented storage system with 612 m2 flat plate collector.
SHORT DESCRIPTION OF THE FLAT PLATE COLLECTOR SYSTEM
Holms and Friends completed a solar water heating (SWH) project for the University of Pretoria (UP) – in 2014. The system will be commissioned in January 2015.
It supplies approximately 570 students with domestic hot water in both the existing Olympus- and new Prinshof residences. A very tight site and construction program, made this installation challenging. In addition, the services of the existing Olympus residences needed to be fully functional at all times. Only 4 hours off time were allowed to move the existing 17,500-litre hot water vessel to its new position and re-connect the hot water service.
CHARACTERISTIC’S OF THE FLAT PLATE COLLECTOR SYSTEM
The 306 x 2m2 collectors for this system were installed on top of the building roofs. The collectors are a total of 612m2 gross area and linked to twelve systems, each feeding a hot water circulation loop on a central duct. On the roof, 2 collectors are always connected in parallel. Two parallel strings are then connected in series, resulting in a thermal length of 12 m. The collectors are fixed to a aluminium railing system which in turn is fixed to a special cleat system on a Craft-lock roof sheeting by Clotan steel. Other than the two pipes per system, there are no roof penetrations which will reduce the need for maintenance.
Each array charges its own hot water storage vessel through a solar charging station. The solar charging stations by LME have a controller UVR 63-1 from Technische Alternative and have a collective heat exchanger capacity of just under 458kWth which load the storage vessels in layers. This is to ensure optimum charging ilo of mixing the water.
The solar loops are charged with a water-glycol solution (20%), which is protection against freezing conditions. In addition the system is designed to handle full stagnation during the sunny summer holidays, when consumption might be very little. This is done through Membrane Expansion Vessels (MEV). As the buffer tanks are high pressure, addition expansion needs to be allowed for. The total capacity of all MEV is just below 5,000-litre.
There are 2 x 2,000- and 8 x 3,000- buffer tanks and 1 x 17,500-litre hot water storage vessel. In addition there is one small 300-litre hot water vessel on the parents home. These are high pressure mild steel vessels without any internal protective coating. As no oxygen gets into the tank and therefore no rust can occur. The vessels are well insulated with rockwool of between 80 – 120mm thickness. In addition, a galvanised metal sheeting protects the insulation from outside moisture and damage. All hot water vessels are installed on a plinth in a plant room.
Through external heat exchangers (i.e. they are not in the tank itself), a different continuous fresh-water supply is warmed and distributed to the individual residences. This results in an indirect loop system, having the advantage that the water in the large storage tanks, heated by the panels, automatically complies with health standards without major maintenance, since it will never be used for human consumption. It is merely the working fluid. The total heat exchange capacity of the fresh water stations supplied by LME is 2,200kWth. This is of importance as virtually all students consume hot water within a timespan of only two hours/day. The existing 17,500-litre tank has no fresh water station.
The water is distributed through pump-circulation, via 20 – 40mm diameter, heavily insulated copper pipes. These are mounted inside the service ducts at each block, supplying instantaneous hot water to all hand wash basins, sinks and showers.
We therefore could not just replace one energy system (electrical resistive) with a renewable (solar water heating) system. The water consumption had to be reduced. This was done by:
- Designing the building to have warmer bathrooms (avoiding heating the rooms with hot water and/or extended shower times),
- Provide an insulated hot water circulation pipe (avoiding long dead leg pipes with the associated cold water run),
- Using low flow showerheads and taps,
- Specifying timed taps (which need to be pressed again to dispense more water) and
- Creating awareness amongst end users.