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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.
A recent UN study reports more than 50% of the global population live in built up urban environments. In fact, with developed nations this proportion soars to over 75%, but as city growth and regeneration continues there is a corresponding aspiration for space. However, this can meet planning resistance as, rightly so, surrounding greenbelt is a protected resource. With our understanding of climate change and the importance of vegetation, there has been a subsequent push to vertical city growth through multi storey buildings instead.
With concrete jungle growth comes a greater demand for green space where people can congregate, relax and enjoy their surroundings. However, the presence of these green spaces within a myriad of high rise structures makes it tricky to ensure usability of the space throughout the year. It becomes necessary to understand the suitability of external comfort to ensure green spaces are optimally designed and located to maximise their function throughout the year, which includes alleviating the effects of flow around nearby high-rise buildings.
Take the case when the wind impinges on the face of a building. Bernoulli’s equation states the air will slow down and the building face pressure will increase significantly. The natural tendency then is the high pressure air will flow towards a lower pressure area which typically exists at the building’s base; an effect called downwash. These flows are regularly observed at the base of tall buildings, especially when surrounded by shorter buildings. Once the downwash reaches the ground level it spreads horizontally with pedestrians experiencing this as sudden gusts. These gusts occur even in conditions which are termed as a ‘light breeze’ by recorded weather data and can lead to micro climates.
A second example of air accelerating is when street layouts are funnelled from a wide to narrow path. This is a regular situation in existing city centre layouts, however this can be considered in new and regeneration schemes. It is also observed in cases where there are large openings at the base of the buildings used to create spaces like a reception concourse. Although visually impressive and imposing, its weakness is the potential for accelerating air.
Too often both situations come together with the effect multiplying, and at times generating hazardous conditions. There have been many cases where pedestrians have complained of difficulty opening doors and comfortably crossing open spaces due to pressure imbalance.
When buildings and sites are being planned, early stage conceptual modelling can measure the risk. Pedestrian comfort analysis is performed to study the conditions surrounding the site under a range of wind speeds and directions. Consider it as a virtual wind tunnel, where a 3D form of the site is massed and subjected to the test considerations. Air speed is measured at points of interest, and this data along with local weather data is used to obtain an annual perspective of the local air speed across the area of interest. Statistical analysis can then be performed to check compliance against various wind comfort criterion. Remember, the brilliance of the model is that the speed feedback can be introduced and the tests rerun to optimise the design.
Now, turning to the popular metric currently in use, the Lawson criterion, which is divided into two parts. Firstly, it covers pedestrian comfort for regular activities like walking, standing and seating. This helps in determining the usability of a location or site against that particular activity. The criterion sets out threshold local air speeds based on the activity, which cannot be exceeded for more than 5% of the year. The other aspect is the safety criterion which stipulates the air speeds which cannot be exceeded even once a year. This ensures that pedestrians and cyclists are not in danger of physical harm from high air speeds.
The impact of urban developments is felt globally through carbon emissions, their internal conditions (as we live and work within them), but also in how we commute and enjoy the space between them. We can make the best of all three through modelling.
IES Consulting’s CFD experts use a combination of IESVE MicroFlo and the 3rd party CFD software package Engys® HELYX®. MicroFlo was developed to provide our customers with a relatively simple and accessible CFD package which can be used by engineers in conjunction with our ApacheSim tool. The Engys® HELYX® CFD package, derived from the OpenFOAM® libraries, allows us to use bespoke tools for performing complex analyses like data centres, external pollutant studies, pedestrian comfort and external flows with heat transfer, etc.
If you’d like to find out more about our CFD Consulting Services, please contact Harshad.Joshi@iesve.com.
In a testament to the talent and high level of expertise of our people, Consulting-Specifying Engineer (CSE) magazine, for the 5th time, have selected an IES member of staff to receive a prestigious 40 under 40 award. This year’s winner is Colin Rees, Consultancy Manager at IES.
The award is given to 40 non-residential building industry professionals age 40 and younger who stand out in personal and professional aspects of their lives. And Colin certainly does. As the longest serving Consultant with 14 years of service, Colin has supported the start-up of two IES office’s, San Francisco in 2007 and Pune in 2010. He’s played a key part in ensuring the sustainability and high performance of many renowned projects across the world and has used his expertise and experience to mentor and train other consultants in the IES Consultancy team to help make it the dedicated, highly experienced team it is today. Read Colin’s full winner profile.
Previous IES winners are:
We pride ourselves in hiring people who are committed to sustainability and passionate about what we do. And in turn we offer a flexible and supportive working environment and the opportunity to work with a team of friendly, interesting and diverse people from across the globe. If IES sounds like a place you’d like to work, then keep an eye on our vacancies and follow @IESCareers on twitter. You can also send in a speculative CV to firstname.lastname@example.org.
You can view profiles of all this year’s 40 under 40 winners on CSE Magazine’s website.
The recent COP21 summit in Paris again threw into focus the challenge of climate change, with urban development being confronted to reduce their energy usage. Simultaneously there is a growing concern on how overheating is severely impacting building performance and occupant comfort. With rising global temperatures being experienced now and significant increases expected over the short to medium term, overheating is a key issue that needs to be addressed. Occupant comfort is still a major concern as is energy use and they are both intrinsically linked.
Modern buildings are well sealed and insulated and in London where outside temperatures are higher than average this can lead to an enhanced need for cooling during the summer both in Residential and Non-Residential properties.
A historic design response to avoid overheating would have been to introduce comfort cooling measures but this brings additional energy and carbon use as well as higher running and maintenance cost. However, contemporary design approaches more frequently look to tackle solar and internal loads through passive design methods that minimise their impact without retrospective cooling measures being required, or where necessary allow ventilation approaches with mechanical cooling capacity to offset the peak cooling load.
Developing a response to climate change has led London to introduce a chapter specific to this in its London Plan. Policy 5.9 seeks to adapt to climate change by directly addressing the overheating and cooling conundrum. As London suffers from the urban heat island effect, retrofit and new build need to prioritise the opportunities available to reduce the cooling load and remove the potential for space overheating.
To further investigate the impact and mitigation of overheating, a new dataset of weather files has been released by CIBSE to dynamically simulate against the 2020’s, 2050’s and 2080’s. These files for London and other UK locations will offer climate change scenarios to benchmark the projected building performance. Additionally, London has a TM49 dataset representing three summers with different types of hot events.
Dynamic simulation can present fast yet detailed parametric datasets offering the ability to compare design options and drive the optimisation of the most beneficial design solutions such as shading, glass type, window-to-wall ratio, mixed mode ventilation, thermal mass, etc. The Greater London Authority have rightly identified an expectation for dynamic simulation to be used to demonstrate overheating performance. Without a robust analysis you can’t rely on the results and lack of good data leads to plant oversizing and operational inefficiencies.
IES Consulting have the experience to help investigate and interpret the impact on your building design by employing these new datasets for retrofit or new design through a parametric modelling approach where a large number of options can be run in parallel to optimise decision making. IES work with you from the concept stage to build a scope of design options and then provide detailed feedback on the best value opportunities. By generating reliable data we help project teams design with confidence, sizing plant correctly to operate at optimal efficiency and minimise their capital costs without compromising on comfort.