Trigeneration: Sydney's white elephant

Matt Wright

The City of Sydney is comparing apples with oranges when it promotes its Trigeneration Master Plan (designed in partnership with Origin Energy and subsidiary Cogent Energy) as cleaner than burning coal in centralised power plants.

Beyond Zero Emissions’ fundamental criticism of the trigen plan has yet to be adequately answered. In May, I pointed out in an article that it’s easy to look better than the coal-dominated status quo, but it would have been more legitimate if the City of Sydney had compared its plan to other possible efficiency upgrades with similar capital expenditure and energy costs.

Now the Property Council of Australia has weighed into the debate with a submission that backs this up. The Property Council suggests that energy efficiency (demand reduction) and renewable energy sources should be the first priorities before trigen is considered.

This suggestion is welcome. Inefficiency in city office energy use is driven by poor building design, and high peak loads when old, inefficient air conditioners are used to compensate for this poor design.

It’s time for the City of Sydney to park the trigen plan, and start implementing energy efficiency measures and building renewables. If it does that thoroughly, it will find trigeneration unnecessary.

Trigeneration plant

Trigeneration gas engine

Later this year, Beyond Zero Emissions will be releasing the Zero Carbon Australia buildings plan. Our extensive research for this plan has indicated much better paths to take than investing in the internal combustion gas engines and associated heat-driven absorption chillers used in trigen.

The argument to date revolves around efficiency figures, including the efficiency of the current electricity grid, the efficiency of the trigen plan, and that of existing building chiller systems (based on their Co-efficient of Performance, or COP: the amount of heating or cooling energy delivered expressed as a multiple of the input energy).

The architect of the trigen plan, Allan Jones, has argued in an article that we should use the primary energy ratio (PER), which takes account of the efficiency of the electricity and grids, not just the COP. This is fair, although our energy flow diagram we developed to make comparisons originally did do this, by factoring heat losses at remote power plants.

Picture: energy flow diagram of trigen versus two other possible efficiency scenarios.

Using the PER (as they do in Europe), Jones said “the average COP of electric chillers in the city is 2.5. However, the electrical efficiency of the grid is only 31 per cent or less in the city so the overall COP or primary energy efficiency is 0.77 or less.”

But this makes them still about the same primary energy efficiency as an absorption chiller used in the trigeneration scheme: 0.7 COP is typical.

Claiming PER of 100 per cent times 0.7 COP for the chillers (as Jones does) only works if you allocate all the heat losses to the electricity generation side of the system by an accounting trick. In fact, as our diagram shows (and the Master Plan also shows) there are still significant heat losses in the trigen system, however you account for them.

Jones pointed out that the absorption chillers have to be optimised to the size of the expected waste heat stream. “For example, if absorption chillers with twice the COP were used this would have the effect of either reducing electricity generation by 50 per cent or rejecting 50 per cent of the waste heat. Neither is desirable or efficient and would significantly reduce carbon abatement which is why the COP of absorption chillers have to be selected to fit the waste heat output,” he explained.

As the waste heat stream goes up and down with electricity generation, then the chillers must fit the normal heat output, to run at greatest efficiency.

For this reason, whilst an absorption chiller could be sized to meet the peak demand of the airconditioning, it is more efficient (and common) to keep the pre-existing chiller available for the mid-afternoon peak requirements. The City of Sydney Master Plan includes this.

The absorption chiller must then be sized for the average load. Otherwise, much of the time it will be running at less than maximum efficiency. A heat pump based vapour compression system, running off grid electricity, can vary its output as needed (as pre-existing units at COP 2.5 do).

But instead of requiring building owners to retain and maintain their old chiller systems as back-up, and still have to pay for installing new absorption chillers, the simpler option of upgrading the old electricity-driven chillers should be considered.

And so we are now comparing apples with apples. Jones only compared the trigen’s absorption chillers to the current building chillers with a low COP of around 2.5 on average. How does it stack up against the alternative upgrade scenario?

Modern vapour compression chillers have a COP of around 6 to 7. If we take it as 6.5 (as in our diagram), their primary energy ratio would be 2.0, as compared to 0.77 (based on the 31 per cent NSW grid efficiency quoted by Allan Jones).

It is expected too, that the grid efficiency will continue to improve. Even without having city buildings source 100 per cent green power, the 20 per cent Renewable Energy Target will be improving the efficiency of the grid.

Sourcing 100 per cent renewable energy would give a primary energy ratio of 100 per cent minus only 5 per cent or so for transmission losses. The primary energy ratio measures energy lost from the source, such as as heat waste in the conversion of fossil fuels to electricity. Solar and wind aren’t fuel-based and nothing is “wasted” if they are not captured; they are renewable.

Further, a significant proportion of energy could be generated by solar panels on rooftops distributed in and around Sydney’s CBD, reducing grid losses just as in the trigeneration scenario.

As the Property Council put it in its submission, “PV generated electricity (i.e. green power) connected to the latest turbocor chillers is more competitive in the current market than the City’s proposed trigeneration system. Our members ask why should the City subsidise a fossil fuel solution over a renewable competitor?”

Cost of gas

Having discussed efficiencies, it’s also worth considering the cost of fossil gas.

As reported in the Sydney Morning Herald in April, the NSW state government warned that ''Bass Strait and Cooper Basin gas supplies are dwindling at a time when the gas export industry is growing at an extraordinary rate.” These are the NSW’s main gas supplies until dirty coal-seam gas can replace them.

The article explained that a “further delay in gas developments in NSW will contribute to a supply shortfall as early as 2014, and could have a direct impact on the state's consumers, through gas supply challenges and higher energy costs.''

The decline in the Cooper Basin supply has led gas supplier Santos to begin producing shale gas in that area. Santos admits that the South Australian wholesale price will be double that of the conventional gas currently sourced from their Moomba gas plant.

The City of Sydney will produce its Renewable Energy Master Plan later this year, and promises to look at sourcing renewable biogas from waste, sewage and so on.

Even if sufficient biogas can be sourced to make its plan run on renewables – no small task – it is hard to imagine that biogas sourced this way will be sold any cheaper on the market than the fossil gas that it is essentially interchangeable with.

Australian gas prices are now being tied to international market prices, which are likely to be high and volatile. On the other hand, even with a higher upfront cost of deployment, renewable solar and wind have zero fuel costs.

In any case, it would be foolish to waste portable, liquid/gas biofuels on power generation, which can be done with solar and wind. Under BZE’s zero emissions scenario, biofuels will be needed for some transport use, and for feedstock for industrial processes like steelmaking, farm inputs and plastics.

BZE’s research has modelled the energy reductions from a number of retrofits to office buildings built in the 1980-2000 period, which is the age of most of our current building stock.

Measures recommended include LED lighting replacement,  fabric upgrades (insulation and low-e window film), HVAC (chiller) upgrades, replacing old IT equipment, and heat pump-sourced hot water.

Based on these efficiency measures, Australia’s commercial city office blocks would commonly see reductions in their final energy use of 70 per cent. In combination with using renewable energy sources, this could give us net zero carbon emissions from buildings.

Even going to just 50 per cent renewable energy, with upgraded chillers, resulted in 60 per cent emissions reduction (compared to current practice). Our very best modeling result for a precinct trigeneration plan was 58 per cent emissions reductions, with no further room for improvement after that.

In fact, the Property Council has outlined the same general approach: efficiency, then renewables.

It’s a much better bet than locking in dirty and expensive gas supplies.

Matt Wright is the Executive director of Beyond Zero Emissions.

Source: Climate Spectator