Boekenhout & Olienhout student residences, University of Pretoria | Holms and Friends: 52-kL low pressure metal vented storage system with 818m2 flat plate collector.

Short description of the metal vented storage system

Holms and Friends completed a solar water heating (SWH) project for the University of Pretoria (UP) – in 2012/13. The company specialises in integrated energy strategies and EPC for large-scale, commercial solar water heating and photovoltaic systems. They are known for the training they present in the renewable energy and energy efficiency sector as well as education for sustainable development.

The system supplies over 650 students with domestic hot water in both refurbished residences.

Characteristics of the metal vented storage system

The 409x2m2 collectors for this Metal vented storage system were installed on top of the building roofs. The collectors are a total of 818m2 gross and linked to eight systems, each feeding a central duct. On the roof, 3 collectors are always connected in parallel. Two parallel strings are then connected in series, resulting in a thermal length of 12 m. Each array charges its own hot water storage vessel through a solar charging station.

The solar charging stations have a collective heat exchanger capacity of just under 600kWth 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) with a total capacity of over 3,500-litre.

There are 2 x 1,600-, 4 x 7,200- and 2 x 10,000-litre buffer tanks. These are steel vessels without any internal protective coating. As no oxygen gets into the tank, no rust can occur. The vessels are well insulated with Rockwool of between 120 – 150mm thickness. In addition, a galvanised metal sheeting protects the insulation from outside moisture & damage.

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 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.

The project included energy efficiency & passive design on the building with the following objectives:

  • Reduce the water (especially hot water) consumption,
  • Reduce the energy losses on the complete metal vented storage system,
  • Create better central control (i.e. remove the then uncontrolled horizontal electrical geysers distributed all over the buildings),
  • Enlarge well insulated storage capacity to avoid peak demand electricity,
  • Reduce running cost and deliver cheaper accommodation,
  • Reduce the carbon footprint by using renewable energy,
  • Create awareness amongst users (students), maintenance teams & management and
  • Support the University of Pretoria brand.

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.


Henning Holm has approval from client to film.


  • Students have warm water in certain sectors for the first time since the residences were constructed.
  • Total warm water pipe length has been reduced by over 700meter.
  • Systems are running for 1 and 1.5 years respectively and the client is satisfied with the performance.
  • Solar array on each block in construction (note expansion gaps).
  • Installation of the 10,000-litre buffer storage vessel.