For many building types, maintaining space conditions will account for a significant proportion of the overall energy consumption. With the effects of climate change expected to increase the external air temperature, buildings will see a shift toward an increased cooling need. If we are going to reduce our carbon emissions whilst minimising running costs then the onus will be on designers to correctly ensure their plant design is optimised. The key will be to accurately predict the cooling energy demand and recognise how a considered system will operate in practice. Not doing this accurately could lead to increased risk exposure when the building is operational. So what considerations should designers be recognising in the early stages?
If we start by looking at the UK compliance route which typically uses a Seasonal Energy Efficiency Ratio (SEER) as a means of determining the cooling energy. SEER figures are intended to represent a weighted system efficiency that takes into consideration performance at varying part load conditions and at varying ambient temperatures. Manufacturer published SEER values are based on a specific set of conditions that in all likelihood will be very different to the range of conditions that a particular building is likely to operate within. While SEER values are useful for designers so to compare a range of products on a like for like basis they do not necessarily reflect the true performance when that cooling plant is then operational.
Typically SEER’s used in UK Compliance calculations will be based on the following standardised conditions:
|Part Load||Weighting Factor(Office Building)||Weighting Factor(Unknown Profile)||Air Cooled Chillers – Ambient Dry Bulb Temperature||Water Cooled Chillers – Condenser Entering Water Temperature|
The operating conditions for different part load conditions are used for benchmarking chillers internationally and do not necessarily even reflect the UK’s climate.
For many parts of the UK the air temperatures assumed for the SEER calculation at a 75% and 100% part load will rarely be observed. If the building is not an office type and the load distribution is unknown then the four part load points will be weighted equally putting an even greater significance on the plant at a high outdoor temperature when the system will be operating a lower efficiency.
The London TRY05 weather file for example has a peak external temperature of 31.8°C and exceeds 30°C for only 10 hours of the year but based on the standard weighting factors a 33% weighting factor should be applied to the performance at this condition
It should be remembered that simulations can be used to establish an appropriate SEER which permits a more accurate and indeed more favourable assessment.
Real Building Analysis
In many instances design consideration is still only given to the peak cooling demand to ensure the selected plant has sufficient capacity to safeguard the cooling plant needs. However it is important for designers to push on and understand the expected demand load distribution within the design. This is simply the case because a building may frequently experience part load conditions that the plant cannot meet efficiently, or its demand is lower than the cooling plant’s minimum turndown, or even the cooling system is an innovative design with multiple components where coolth is captured from many sources.
Take the example of when the cooling load demand is lower than the chiller’s minimum turndown, it will typically cycle on and off with this behaviour leading to inefficient performance. This in turn applies unnecessary stress on the plant which may result in faults and in places failure, but most certainly increased maintenance costs. Who will bear this burden? A building owner should not feasible expect this process after their investment has already been made.
To solve this step then the design should consider how the range of the cooling plant output can be extended through a modular chiller design. The chart below compares this range where the chillers can operate continuously for a single chiller and multiple chiller configuration.
While the chart above demonstrates that only a lesser proportion of part load conditions will lead to chiller cycling, this part load range can still be a common condition especially if the chiller plant has already been oversized.
As well as increasing the operational range of the chiller output, sequencing can be utilised to maximise the cooling plants operational efficiency by either running single or multiple chillers to achieve the best operating condition. The illustrative example below demonstrates a chiller’s COP at varying part load conditions. Alongside the COP, the load frequency is displayed indicating how often the cooling load falls into a particular range. This illustrates how sequencing multiple chillers can be utilised to provide smaller part loads efficiently.
The following series of charts illustrate how implementing multiple chillers and with sequencing can meet building loads more efficiently. The green bars represent the frequency a particular load condition occurs and in the example we see a significant number of hours where the cooling demand is relatively low. The grey line illustrates the systems efficiency at that part load condition.
The first example represents a single chiller serving the building where it operates at a poor efficiency across many hours. The SEER in this instance is 3.68.
In the second example the same cooling demand is now met by two equally sized chillers which allows smaller loads to be met more efficiently than previously. The SEER now increases to 3.98.
The final example uses two unequally sized chillers to go further in efficiently meeting the building loads. Now the SEER has risen again to 4.05.
The above examples are illustrative but are intended to demonstrate how the impact of plant selection and control can influence the performance of a building.
The efficiency of a building cooling plant is sensitive to a number of considerations including water loop temperatures, part load conditions and the ambient temperature, all of which continually change across the year. Only within a dynamic simulation can the building plant be modelled in the required detail to predict the true operation. With feedback from the dynamic simulation designers can confidently understand the HVAC plant operation and its responses to the pressures put on the building. Modelling the intended operation during the design stage will help identify potential risk far in advance to ensure an efficient and optimised plant.