22 MAY 2011 Climate Control News

Case Study

The paradigm shift to patient and family-centred care and the inability of ageing hospitals to accommodate these latest

models, have all contributed to the current worldwide boom in hospital redevelopment.

The evolving ‘new breed’ of hospital is welcoming, less threatening to patients, integrates sympathetically with the local community and embraces evidence-based design principles. These new hospitals are now more integrated into their local communities than ever before and with this attitudinal change comes new pressures for our ‘new-look’ hospitals to become exemplary neighbours.

By nature of their function, hospitals are major users of power and water, not to mention historically being major environmental polluters. Melbourne’s new Royal Children’s Hospital (RCH) is not only regarded as a world class facility by virtue of its design and cutting edge medical technology, but it also incorporates the very latest building services technology and sustainability initiatives. The new RCH site is located in parkland and directly across the road from Parkville residences, which provided even greater incentives to ensure that the new world class complex qualified as being as ‘green’ a neighbour as possible.

susTainaBle TargeTsSignificant environmentally sustainable design (ESD) targets were defined at the outset for the new RCH by the Victorian government in relation to the reduction of energy, carbon dioxide and potable water use. This strategy ensures that this new healthcare facility significantly raises the sustainability bar for other planned major healthcare facilities. The two key RCH initiatives implemented include a

blackwater treatment plant (a hospital first) and secondly trigeneration, with both systems playing a significant role in meeting and exceeding ESD targets.

The trigeneration plant and technology engineered by Norman Disney and Young (NDY) is undoubtedly one of the cornerstone ESD initiatives implemented for the new RCH.

Apart from the carbon reduction benefits provided, it will also produce electricity and heat energy with a system efficiency of around 78 per cent. This higher efficiency level is far in excess of the 35 to 40 per cent system efficiency associated with grid power. The poor efficiency of grid power is largely due to the traditional coal-fired generation plant currently employed as well as transmission and distribution losses which account for around eight per cent.

all enCOMPassing answerRCH has a heat-led trigeneration system comprising of two 1160kW gas reciprocating engines and two 1267kWr two-stage absorption chillers. The system generates 25 per cent of the RCH base building electrical demand, plus a contribution to chilled water and heating hot water for air conditioning and a heating contribution to domestic hot water.

Carbon reduction from the trigeneration system is around

37 per cent, with a further 10 per cent reduction in carbon emissions from the use of a renewable technology biomass boiler (burning compressed timber pellets from forestry waste) and solar preheating of domestic hot water serving the inpatient unit.

The two trigeneration engines also contribute to the 100 per cent overall standby capacity which operates in the event of a grid power failure.

There are significant environmental gains from the on-site generated electrical contribution which offsets the need for the equivalent capacity in much less efficient grid power and effectively reduces the electrical demand by 25 per cent.

Further benefits accrue from the recovery of otherwise wasted heat for use in space heating and cooling which means that the equivalent capacity of heating and cooling is saved from needing to be generated via gas fired boilers and electrically powered chillers. While trigeneration is not a ‘replace all’ solution, it does mean that the overall capacity of the conventional electric chillers and gas boiler plant can be somewhat reduced in capacity.

sO whaT exaCTly is TrigeneraTiOn?Essentially, trigeneration is a three-way complex central plant system employed to generate electricity, heating and cooling.

The system generally comprises multiple units of natural gas-fired engines or gas turbines, the latter being more suited for larger installations. These prime movers are close-

Trigeneration for a new generation of hospitalA trigeneration system installed at the new Royal Children’s Hospital in Melbourne generates 25 per cent of base building electrical demand, Norman, Disney and Young’s Keith Davis reports.

The royal Children’s hopsital and its place in Melbourne’s surrounds.

keith davis of ndy.

diagram 1:The rCh energy flow chart.

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Case Study

coupled to alternators which generate the electrical component. The heat generated from the prime-mover combustion, which ordinarily would be discharged to atmosphere via the exhaust system, is captured and used to generate heating hot water and chilled water for air conditioning. See Diagram 1.

The RCH chilled water requirements produced in part from the trigeneration waste heat is generated by the absorption chiller plant, the operation of which is not dissimilar to that of a gas-fired camping fridge. The cooling cycle involves compression and evaporation phases and a chemical process plus an exchange of heat. Steam or hot water enters the absorption chiller and produces chilled water plus a quantum of waste heat which is then rejected to atmosphere via water-cooled heat rejection plant at roof level.

The heat rejection plant requires a significant quantity of potable water for cooling and this is achieved by using recycled grade A water from the hospital’s blackwater treatment plant recycling what would ordinarily be lost to the sewer.

Logically there is little need to run a trigeneration system when the requirement for space heating or cooling is significantly

reduced. For this reason it is essential to ensure that the plant is appropriately sized. If a heat-led system is oversized, at low electrical and/or thermal demand the trigeneration will shut down and any benefits of having the system are effectively lost.

Parameters used to assess the economic feasibility of a potential trigeneration system must address: hours of operation, lifecycle assessments and carbon reduction potential.

geT The lOad righTThe new RCH is unique in nature and comprises both a category 1 hospital and large medical research facility in the form of the Murdoch Children’s Research Institute, plus retail and significant undercover parking all situated on a large campus totalling 114,918 m2. Each area was grouped according to their varying schedule of operating hours and load conditions.

This uniqueness complicated the benchmarking of the new RCH against past hospital developments.

To address this issue, NDY was able to develop individual load and operating profiles for each functional space such as offices, IPUs, ambulatory care, clinical and laboratories. Previous NDY project experience plus international benchmarking were used to

Construction of the facade to match the artist’s

impression.

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24 MAY 2011 Climate Control News

Case Studyprovide these profiles. These were then applied on a pro-rata area basis to establish the overall building model.

This effectively meant that the load characteristics for the building were developed from scratch and modelled by constructing the predictive load profiles for the building electrical demand, cooling demand and heating demand over a year in order to take account of the seasonal effects.

The modelling also took into account the operational diversity across the various departments.

For example the North Building (IPU) will have patients and staff occupying the space 24/7 which will require cooling/heating requirements during the night. The West Building (offices), however, will be predominantly 8am to 6pm hours of operation with minimal loads overnight.

insTallaTiOnA cogeneration/trigeneration system is a very dynamic system that relies on advanced controls to manage the efficient and practical operation of the gas generator sets, absorption chillers and heat exchangers.

Accurate modelling of such a system requires detailed inputs of loads and operating conditions over the entire development. As this project is still under design (with some components undesigned at the time of this report), our analysis can only provide a ‘best guess’ overview of the trigeneration system’s typical operating hours and achieved energy reductions.

In order to simulate the effects that many variables may have on the trigeneration plant’s operating hours, carbon/energy reductions and payback period, assumptions have been made based on manufacturer’s data, common benchmarks and the likely operating conditions. These assumptions are indicative only to allow comparisons to be made between various sizes/plant configurations. See sidebar.

The CHW and HHW piping lengths are relatively short, under 20 metres, as the ring mains it connects to are directly above. However the CCW piping length is

aBOVe: large, open, glassed areas make a mechanical engineer’s dream.

aBOVe: The heart of trigeneration, the gas-fired engine.

The absorption chiller. Other side of the absorption chiller.

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Case Study

approximately 120m as the heat rejection is eight stories above.

Comparing installation time, to commission point, and ease of install for absorption chillers and trigeneration as compared to a regular chiller system is interesting.

Electrical chillers are typically precommissioned by the supplier off-site, making on-site installation and commissioning times comparatively shorter. The installation and commissioning time for trigeneration and absorption chiller systems are much longer, as a result of the multiple supplementary systems such as heat exchangers, pumps, sensor, controls, silencers etc., that need to be installed and interconnected.

The reciprocating engines also form a part of the standby electrical system, leading to additional connections with change-over switches and redundancy, start-up times and shut-down of associated systems.

Due to the specifics of the RCH facility systems, the trigeneration system is still undergoing commissioning testing and combined services testing, where as the electrical chillers

are currently commissioned and running to allow the commissioning of other systems. The full extent of the trigeneration system commissioning has yet to be reached.

POsiTiOn On TrigeneraTiOnAn accelerating of the supply authority network upgrades combined with an increase in the buy-back tariff to a more commercial rate, would inevitably see a significant growth in the size and coverage of privately owned and managed trigeneration plant.

Such growth would in turn significantly reduce consumer dependence on our traditional coal-fired power stations and will produce a significant positive step toward carbon reduction across the nation.

Keith Davis is director of health services at engineering consultancy Norman Disney and Young. Responsible for the strategic direction of NDY’s healthcare group in Australia and New Zealand, Davis brings extensive hospital and laboratory experience over 35 years as a consultant in South Africa, London and Australia.

PrOjeCT inPuT VariaBles are:Gas tariff (c/MJ): 0.70Export electricity tariff (c/kWh): 2.00Off peak electricity tariff (c/kWh): 3.00Peak electricity tariff (c/kWh): 11.50Maintenance costs (c/kWh): 1.20Peak demand savings ($/kW/month): 5.00Electrical output: 38.9%Usable waste heat: 37.7%Thermal output: 95.0%Central boiler efficiency: 80.0%Overall efficiency: 76.6%Electrical chiller COP: 6.00Absorption chiller COP: 1.20Turn down of generator: 60%Turn down of absorption chiller: 60%Cost per kW of trigeneration system for payback analysis ($/kW): $1000

aBOVe: in order for the trigeneration system to be operational, there must be sufficient electrical and thermal demand from the site, i.e. when the blue and red lines are sufficiently high. when either of the electrical or thermal demand drops too low one, or both, of the trigeneration units will turn off.

aBOVe: Operation of the trigeneration system demonstrated by the green and yellow lines with trigeneration heating and electricity generation profiles.

aBOVe: Total annual heating, cooling and electricity energy profiles before trigeneration. The electrical load profile does not include the electric chiller loads, these loads are on a separate substation which is not connected to the plant.

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