9 October 2013 – Last week’s article on recent research at RMIT comparing the performance of point of use instant electric hot water systems with a central gas ring main system in two apartment buildings certainly provoked some constructive debate.
As some responses pointed out, there is a balance between energy used to deliver the hot water, storage losses and distribution losses. As well, the level of usage is a key factor. Combustion/conversion efficiency, cost per unit of input energy and greenhouse gas emissions per unit of input energy differ for gas and electricity (and over time). So the cross-over point for performance varies for each factor.
The response that referred to the option of an EcoCute heat pump HWS touched on a particularly interesting aspect. With a coefficient of performance of four or better under a wide range of conditions, this technology dramatically reduces the significance of storage and pipe heat losses, as each unit of heat lost only wastes a quarter as much electricity.
So a high performance central heat pump, maybe with some onsite photo voltaic generation, could indeed produce some interesting results.
Similarly, a high standard of pipe insulation could significantly shift the cross-over point in favour of the central solution.
I recently noted that the Building Code calculation of optimum pipe insulation assumes that high levels of insulation (above about 25 mm) cost over $34 per metre. Clearly, if lower cost insulation could be used, better insulation would be economic and central system efficiency could be much improved.
For me, the interesting issue was not whether one option was always best, but that different options were preferable under different conditions. When distribution and storage losses were high and actual hot water usage low, point of use solutions were preferable, even when using a higher greenhouse impact and more expensive energy source.
This is just one example of the importance of systems thinking in design. In building projects and many other applications, optimising the system can deliver surprisingly large energy and resource savings as well as cutting both operating and capital costs.
I thought it might be useful to highlight some other examples of this important approach.
Systems thinking with water treatment at 60L
When I was involved in the 60L Green Building project, the preliminary design for the rainwater treatment and supply system would have used a lot of energy. This was a fairly simple system: motor, pump, filters, pipes, UV lamp and turbidity sensors. I discussed this with the designer, and asked a couple of key questions:
- Where is all the pressure drop occurring that requires such a large pump?
- Why does the pump run continuously?
It turned out that the large pressure drop was mainly across the proposed sand filters. Switching to oversized cartridge filters dramatically reduced the pressure drop, and reduced water wastage due to backwashing. This was handy as we didn’t have a very large roof area for water collection. The cartridge filters were also recyclable at end of life. Also, slowing the flow rate reduced flow resistance in the pipes.
We also looked at the operation of the pump. One factor here was that each time the UV sterilisation lamp was switched on and off, its life was shortened by an hour or so. The key to savings was a smart control algorithm and use of the turbidity sensors and on-site weather station to manage the pumping. The pump was set to run if it rained, and to run each night until the sensors determined the water was clean.
The pump was also set to wait at least an hour after stopping, before starting again: this meant the elapsed lifetime (not operating time) of the UV lamp was at least as long as if it had run continuously. This saved energy from both pumping and the UV lamp, while lamp replacement costs remained the same over time.
Since there was a requirement for regular testing of water quality at outlets, we also had a cross-check on water quality. This system has used under 10 kilowatt-hours/day – about a kilowatt-hour a year per square metre of floor area.
We could have done better, as the plumber managed to use a lot of high resistance right-angle bends in the installation. And today more efficient, smarter motors and pumps seem to be available.
Split system aircons
Another interesting “central versus distributed system” example from 60L was the use of split system airconditioners instead of a central system. I’m not arguing that this approach is always better, but it has certainly contributed to 60L’s very low energy use (around 80 kWh a sq m annually for building and tenant usage).
To set the context, 60L is a fairly small building (3225 square metres) but it has about 15 separate tenancies of varying size. The kinds of tenants who occupy the building also tend to work very varied hours. So individual units controlled by each tenant offered flexibility and tenant empowerment. Their energy use is also charged directly to each tenant, so they get feedback on their heating and cooling energy use.
The building also relies on natural ventilation much of the time. Another benefit was that the split systems were much cheaper than a central system, so we had more money to spend on other energy saving features.
More energy is used by fans and pumps than to actually heat and cool an
office building in most of Australia
Many argue that central chillers are more efficient than modular units. But many HVAC (heating ventilation, and airconditioning) studies suggest that more energy is used by fans and pumps than to actually heat and cool an office building in most of Australia.
It is also common to find heating and cooling systems “fighting” each other, and for significant reheat energy to be used.
So you need a very efficient central system to match the efficiency of a good split system. Indeed, if the best 5-6 star split systems are used, their COPs can be 5 to 6, including fan energy, And their inverter controls provide very flexible performance. How many central systems could match that on a “delivered heat and coolth” basis?
Another interesting factor has been that, if a split system fails, it only affects a small area within the building. It can be easily replaced by a non-specialist contractor. And control system complexity can be limited.
On the other hand, motor and fan efficiency for air distribution has improved dramatically, while chiller efficiency is also improving, with some remarkable performers now available and smart management systems becoming commonplace.
And we haven’t started on the building envelope …
I haven’t even begun to discuss the complex interactions between building envelope thermal performance and HVAC energy use, but the building is part of the overall “comfort services” system!
So, just as with hot water supply, careful system analysis can lead to very interesting solutions and potentially large energy savings for HVAC.
Systems thinking is becoming critically important in all areas of energy use. For example, in industry, the Energy Efficiency Opportunities scheme requires firms to develop models of energy and material flows through their major systems.
This process had led to identification of numerous exciting and, often, surprisingly large and unexpected energy savings. It would be great to see articles from others on application of systems thinking in their work.
Alan Pears is adjunct professor at RMIT and director of Sustainable Solutions Pty Ltd