Solar Thermal Engineering vs. Conventional Mechanical Engineering

I recently received the following question from a military engineer…

One point I do not see in the calculations so far and begs the point, since they’re using college dorms and barracks as the example facility type, most training barracks soldiers overload hot water capacities long before the daily hygiene events are complete. Simple reason is all soldiers within a platoon act as a single entity and all must cycle through the showers about the same time. Is there a factor that could or even should be employed to accommodate this mass usage drain on hot water? I would assume solar would only supplement a massive (and nearly instant) water heating requirement to meet those “mass” events.

This a very good question and points to a significant difference between solar thermal engineering and conventional mechanical engineering.

Solar thermal design deals with the total energy usage in a system.  Since we have no control over sunshine, and we can’t turn the “burner” on when we need more, we must deal with gathering what is available on a given day, and introducing that energy into the load.

Conventional ME deals with the capacity, or recovery rate of the boiler, and the HW storage volume to meet the highest demand.

The solar is on the upstream side (cold side) of the conventional system and doesn’t know anything about  the water heating system, such as whether it uses gas or oil or electricity, or what size the storage tank is.   It contributes all the energy it can at any give time and leaves the rest of the energy to the conventional system.  It is always secondary, never primary, regardless of what the solar fraction is.

While good solar design emphasizes total energy (per month) over capacity (energy/hr), special cases as described above allow tighter integration between between the solar and conventional systems.  Solar output heat exchangers have to be sized for two sometimes conflicting requirements.

The first is the total solar energy that can be stored in the solar tank, and the duty cycle to remove that energy.  For example,  the typical design duty cycle is 4 hours – 2 in the morning and 2 in the evening.  The exchanger must be sized to remove all the energy in 4 hours. This does make the solar equipment relate to capacity, not just total energy.

In the current example, we may need to use a different duty cycle.  If it is 2 hours total – 1 in the morning, and 1 in the evening, then we can double the size of the heat exchanger.  It is important for the design engineer to understand this dynamic and work with the manufacturer to accommodate these situations.

The second requirement for the solar system is the exchanger pipe sizing.  If the Barracks CW line is 2″, then the solar exchanger should match that line size at ~ 5fps flow.  Sometimes the line size requirement will override the heat exchanger area.  A very large CW line and a relatively small solar system will make the heat exchanger be oversized.  This situation results in excess capacity, not undersized capacity.

So in summary, designing for the capacity of a system (boiler size and HW storage size) are part of the conventional mechanical engineering, ignoring solar as if it were not there.

The solar thermal system, on the other hand, is sized for total energy over the year, but must have some components (specifically the exchanger) sized to match the load duty cycle and the CW delivery system dimensions.

Regards,
Dr. Ben