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FRIDGE AND FREEZER DOORS

It has always been a staple of energy training related to catering that the doors of fridges and freezers should have tight seals and effective closers, and that walk-in freezers should have insulated strip curtains to supplement the proper door when it needs to be kept open temporarily. Most of us would assume that this advice relates to preventing the ingress of ambient air, but that’s not the whole story. When room air gets into a freezer, something like a quarter of the energy needed to cool it down goes into condensing and then freezing the water vapour it was carrying. The amounts involved are not huge: something like 0.02 kWh per cubic metre of air overall. What is significant is that the internal vapour pressure will plunge. So even after the door is closed, ambient moisture will pour in through any gaps in door seals, adding continuous cooling load as the condense-freeze process continues. Meanwhile the resulting ice build-up will be clobbering the energy performance.

It’s atmospheric moisture that you need to keep out.

Keeping a sense of proportion

This web site doesn’t usually cover domestic energy saving but the topic is of indirect relevance when one is conducting staff energy awareness training. Learning about how to cut energy costs at home is one of the benefits to staff  of participating in such programmes.

At the time of writing UK energy prices are increasing dramatically and the media (as ever in such circumstances) are awash with energy-saving tips, many of which are trivial or patronising. As part of any awareness-raising programme it could be useful to steer people away from irrelevant time-wasting ideas and towards things that will actually make a difference. In this article I’ll put some numbers to some of the advice that’s currently doing the rounds. These are rough-and-ready estimates based on a lot of simplifying assumptions and the prices I will use are £0.08 p/kWh for gas and £0.30 p/wWh for electricity.

Tip no. 1: when cooking, avoid opening the oven door to inspect the contents

The argument presented here is that the hot air will escape and more energy will need to be put in to compensate. Let’s look at that: the capacity of the oven will be of the order of 60 litres. Let’s say all the air in the oven is replaced with room air. 60 litres of room-temperature air will have a mass of 0.07 kg. With a specific heat of near enough 1 kJ.kgK,  and supposing a temperature rise of (say) 180 degrees, that implies 0.07 x 1 x 180 = 12.6 kJ = 0.003 kWh, or one-tenth of a pence wasted. Verdict: bonkers.

To put that in perspective, it’s the equivalent of preheating the oven for 4 seconds longer than needed. But even preheating the oven prematurely isn’t a huge deal. Once up to temperature it will very likely dissipate something of the order of one kilowatt (costing 30p per hour) so ten minutes idle costs only about 5p.

Tip no. 2: don’t boil more water than you need

Suppose you boil 0.5 litre more water than you need. With a specific heat of 4.2 kJ/kgK and assuming cold supply at 10°C, the extra heat supplied is 0.5 x 4.2 x (100-10) = 189 kJ = 0.05 kWh or 1.6 pence worth. Verdict: trivial.

Tip no. 3: use a shower rather than a bath

Let’s look first at the cost of a bath using gas-fired hot water. I’ll assume 100 litre (kg) cold feed at 10°C and bathwater heated to 45°C. At a specific heat of 4.2 kJ/kgK that needs 100 x 4.2 x (45-10) = 14,700 kJ of net heat. Assuming 80% boiler efficiency that equates to 18,375 kJ gross , i.e. 5.1 kWh or say 40 pence.

Contrast that with 10 minutes in an 11kW electric shower: that’s 11  x 10/60 = 1.8 kWh, costing 55 pence, a bit more than the bath. Verdict: pointless

What about a shower fed from the gas heating? Suppose it’s a combi boiler with 16 kW water-heating capacity operating at 80% efficiency (ie 20 kW input) again for ten minutes. That would use 20 x 10/60 = 3.3 kWh of gas, costing 27 pence (13 pence less than the bath). Verdict: unexciting

Tip no. 4: turn off unwanted lights

Let’s take for our example an LED lamp rated at 10 watt. That will cost about 3 pence per hour to run but unlike ovens, kettles and baths, one tends to have a lot of them and use them continually so their cumulative effect in a  household could be relatively costly. Eight such lamps run on a daily basis for four hours more than needed would add 8 x 4 x 10 x 365 = 116.8 kWh per year, costing an extra £35 per year. Verdict: do it

 

 

 

 

Hybrid alternating-direct electricity supplies

DATELINE 1 APRIL, 2022:  Thanks to reader John S., who alerted me to a company which has been trying to raise finance to develop an alternating-current (AC) rechargeable battery technology based on its concept of a ‘biode’, a battery electrode that switches continually between being an anode and cathode. While an AC battery is an intriguing concept in its own right, it is the application to mains supplies that interests me more.

Because the electrons in an AC supply continually flow into and out of the load, rather than continuously in the same direction as with DC, the cumulative net current flow over a given time interval is actually zero. The only reason that energy is consumed is that the voltage also alternates: power is voltage times current, and negative voltages multiplied by negative currents give positive power. But what if one adapted the biode principle for mains power? If you were to blend DC voltage with alternating current you would get alternating power, with the customer feeding back as much energy to the grid as they absorbed, 50 times a second (when negative current is multiplied by positive voltage).

At the power station this blended alternating/direct (BAD) supply could cause problems because obviously the returning energy is never going to recombine CO2 from the atmosphere into fuel (that’s entropy for you). Admittedly a wind turbine for example might be more reversible, and here the returning power could perhaps be absorbed with the blades working half the time as a fan. However, there is another possibility. If we think about existing three-phase AC distribution networks they already work with a net current flow of zero, which is why a star-connected load does not need a neutral wire. That gives us the germ of an idea. If BAD distribution systems were two-phase rather than three-phase, half the loads could be on one phase and half on the other, and they would take turns to feed each other. Alternatively, by dropping the AC frequency to 0.00001157 Hz (one cycle per day) and adopting seven-phase distribution, you could spread the load between customers over the course of a week.

These ideas are going to need massive investment, and eliminating power stations would occasion huge disinvestment, but this dichotomy is entirely in line with the “net zero power” philosophy I have described. Alongside the technical breakthrough we can expect major innovations in financing, also based loosely on the biode principle: novel bank deposits that fluctuate between credit and debit but average at nil (so-called alternating current accounts). I shared these thoughts with Extinction Rebellion (motto: “stick the man to it”) and they confirmed it makes net zero sense.

Pipework insulation

MISSING insulation on hot pipework is not just a waste of energy and money. It can cause overheating of the space it occupies, may compromise delivery temperatures, and may even constitute a scalding hazard.

Allowable heat losses are stipulated in British Standard 5422, which lays down the requirements for compliance with building services compliance guides.

VESMA.COM provides a free on-line calculator which enables you to check whether a given thickness of a particular insulant is likely to be adequate.


STOP PRESS we are running a two-hour technical briefing on pipe, tank and duct insulation presented by Chris Ridge of the Thermal Insulation Contractors’ Association on 7 April, 2022. Details here.

Pre-audit desktop analysis

THE ANALYSIS TECHNIQUES that underpin energy monitoring and targeting have important applications in the search for energy-saving opportunities. A good energy audit doesn’t start with a checklist and a clipboard: it starts with some desktop analysis. Here’s how…

Regression analysis, in which we establish the historical relationship between consumption and its driving factor(s), can give us clues if we see anomalous patterns. Does consumption appear to be weather-related when it shouldn’t be, as in Figure 1? Does it fail to respond to production throughput (as in Figure 2) when logically it ought to vary? Do we seem to have unreasonable levels of fixed consumption?

Figure 1: electricity consumption on this campus was strongly weather related even though it had gas-fired main heating. The relationship should have been a horizontal line rather than sloping. Students were using portable electric heaters in their rooms
Figure 2: electricity consumption on this log-chipper did not fall with lower throughput as one might expect. The machine had high losses and was running continuously although logs were being fed through only occasionally

Regression analysis also enables ‘parametric’ benchmarking which is a simple but more effective variation on the theme (see separate article).

Cusum analysis meanwhile shows us whether past performance has been consistent, and if not, when it changed plus (when combined with regression analysis) in what manner. Did we add (or lose) some fixed demand? Or did sensitivity to a driving factor change? (Read more about cusum here).

Next, the concept of expected consumption enables the computation of ‘performance deficit’, meaning the absolute quantity of energy that we are using in excess of achievable minimum requirements. When translated into cost terms this gives us a clear view of where our most valuable opportunities lie (read more about performance deficit here).

And finally we could add visualisation of fine-grained consumption patterns. But that is costly. Everything else can be done with information collected at weekly intervals.


For training on energy analysis follow this link

Digital twins

Last week I attended a thought-provoking presentation on digital twinning (DT) by the energy manager at Glasgow University, which has built digital twins for five of its buildings. It’s not a topic I know much about but I was interested because, going by what it says on the tin, it sounded like potentially a good tool for what I would call ‘discrepancy detection’ as a way of saving energy. In other words, spotting when a real building’s behaviour deviates from what it should be doing under prevailing circumstances, which will nearly always incur a penalty in excess energy consumption. The other potential benefit of DT to my mind would be the ability to try alternative control strategies on the virtual building to see if they yielded savings, and what adverse impacts there might be on service levels. This would be less intrusive than the default tactic of experimenting on live occupants.

Unfortunately I came away with the impression that we are still a way off achieving these aims. The big obstacle seems to be that DT is not dynamic – it only provides a static model. That surprised me a lot, and if any readers have evidence to the contrary, please get in touch. Another misgiving (and to be fair, the presenter was very candid about these issues) was the cost and difficulty of building and calibrating a detailed virtual model of a building and its systems. Then there is the question of all the potential influencing factors that you cannot afford to measure.

My conclusions are in two parts. One is that simulating the effect of alternative control strategies would have to be done with software short of a full DT implementation, in other words, using much-simplified dynamic block models. The other is that discrepancy detection is probably still best done with conventional monitoring-and-targeting approaches using data at the consumption-meter level, with expected consumption patterns derived empirically from historical observations rather than from theoretical models.

Boiler sequencing

This bank of four boiler modules is operating at part load and infra-red imaging confirms that the left-hand module, which is shut down, is not losing heat to atmosphere thanks to an automatic flue damper which prevents cold air being drawn through it.

The other three modules are sharing a relatively light load by running at low output. This tends to incur less loss than operating one or two units at high fire, because the exhaust temperature is lower at reduced burner output.

Testimonials for energy monitoring and targeting training

Over the years I have trained hundreds of energy managers and consultants in the principles of energy monitoring and targeting. Here’s what some of them said…

  • Good combination of things I already knew and things that were new to me. Very engaging and I enjoyed the practical aspects and group involvement. – Jordan Harrison
  • Excellent course with Vilnis Vesma. Provided very useful and key techniques for M&T. Very engaging course as well, with regular exercises. One of the more fun and straightforward course I’ve been on! Thank you Vilnis. – GYE
  • The content is absolutely up-to-date and relevant to my role as energy expert and manager for the industry. All examples for were taken from practice, making this training even more interesting and trusted. It was not my first contact with deviation and CUSUM analyses. But this training brought me new and fresh perspectives. – JLS
  • Great course – RC
  • Great training, very interactive throughout – JW
  • A comprehensive course, packed full of useful nuggets of information – Tim Dennish
  • It’s so refreshing to find a lecturer that is firmly anchored to the ‘real’ world – David Parsons
  • Very useful course with lots of practical applications – Laura Storey
  • A simple approach to a complex subject! – David Roberts
  • Imperative knowledge for all energy managers – Levi Wong
  • The workshop was very worthwhile for someone like me whose business is energy saving products but not familiar with degree day concepts – Len Stevens
  • Excellent day giving basics of energy management – Bruce Claridge
  • A comprehensive workshop for people tasked with energy monitoring & targeting – David Enever
  • I never really understood degree days- and now I do! – Stuart Spencer
  • Vilnis delivers an enthusiastic seminar on energy metering suitable for professionals in the trade and company energy managers – Chris Steer
  • A much more useful way to spend £250 than simply paying your energy bill – Philip Wanless
  • Thought provoking – Ted Bradley
  • Very laid back and made to feel comfortable, being my first course
  • The most beneficial 1 day event I have attended
  • A very useful course with clear and concise information. Essential for any Energy Manager – Jon Farmer
  • Opened my eyes to the amount of added benefit that can be achieved with Degree Day data – Matthew Arnold
  • An excellent review of how to make the most of energy data – Gareth Ellis
  • Great course especially for engineers that use regression & cusum analysis, on a regular basis
  • I found the degree day training one of the most comprehensive and useful experiences of my career. Essential for anyone who needs to know about energy monitoring & targeting – Paige Hodsman
  • Vilnis is a polished presenter and makes a dry topic quite interesting + useful. Well done – Ed Farmer
  • Vilnis has many years of practical experience of energy monitoring & has a passion for sharing his knowledge; very refreshing – Catharine Bull
  • A well structured, thorough and valuable course ideal for businesses keen to save energy – Haydn Young
  • An excellent course for any energy manager wanting to make the most of analysing their consumption data in a meaningful way. Presented in plain English and exercises to help you apply your knowledge – Janette Ackroyd
  • I believe that the knowledge I have gained from the training day will be of great value to my clients and allow me to maintain a longer relationship with them – Laurence Fitch
  • Vilnis really knows his stuff. Excellent course. Highly recommended for anyone interested in monitoring + managing their energy consumption – Simon Hooper
  • A very useful and full day’s training on energy management & targeting. Knows his stuff – Nicholas Smyth
  • A fantastic course full of very useful techniques – Steve Ray
  • Fantastic, very powerful techniques to analyse consumption patterns – Denis Brennan
  • Good use of a day’s time and money – Adrian Evans
  • Within the first hour of the course I was itching to get back to work to put what I was being taught into practice – Adrian Stone
  • Excellent & very well designed – Jonathon Moffat
  • A focused, very practical and extremely useful day. Highly recommended – Nancy Higgins
  • Really targets the essentials – Neil Alcock
  • Gain more, use less. Fly Vilnis – S. Brown
  • Found potential savings the very day after attending the course – Stephen Middleton
  • Very well organised training with excellent presentations. Clear, precise & educational – Julia Clarke
  • The worked examples gave me ideas for analysis using data that was relevant to my organisation – Neil Fletcher
  • Enlightening energy analysis – Andrew Heygate-Browne
  • Excellent, well planned, well paced, inclusive and informative. Most helpful. Thank you – Alan Measures
  • Excellent course, well worth attending – John Taskas
  • All staff should attend this course – Mark Harrison
  • A course which I expected to be boring turned out to be quite interesting – Mick Morris
  • A very useful and extremely interesting course that was well presented – Robert Benson
  • Very interesting and thought provoking course – Avis Street
  • A very structured and well paced training event – Alan Asbury
  • Made concepts which had previously seemed very technical easy to understand – Charlotte Lythgoe
  • A teaching on the fundamentals of M&T that makes M&T simple – David Charles
  • A thorough understanding to the science of efficiencies! – Simon Mansfield
  • excellent, well balanced course and of practical use – George Zych
  • A comprehensive interactive course essential for all energy managers in every sector – Neil Bradley
  • Cusum isn’t the complex, scary thing I thought it was – Rebecca Taylor
  • Brilliant course- very relaxed delivery with practical examples making the material & subject easy to consume – Dave Belshaw
  • Has completely changed the way we analyse our projects going forward – David Dunbar
  • Lots of useful technical information presented at a good pace, and in a very accessible style
  • An excellent day, very sophisticated and detailed presentation for ‘aficionados’ of energy management – David Bradshaw
  • A very useful course, I wish I’d done it five years ago – Phill Windson
  • Excellent day- thanks Vilnis
  • If energy management is new to you then this is the course for you. It’ll get you on the right track straight away. All you could ever need to know- and more!
  • Excellent course that presented some very powerful energy saving techniques – Daniel Jones
  • This will completely revitalise our energy M & T – Donald French
  • armed with these skills you can tackle targeting with confidence – Gary Cooper
  • Excellent, knowledgeable presentation of simple but effective principles and techniques – Richard Ansell
  • Great course to understand the practicalities of energy M & T. – Tom Yearley
  • For once, a course I can actually put into practice at work – AD
  • A comprehensive course, well delivered
  • The course was very good, especially the section on setting targets and making them ‘aggressive but achievable’– AV
  • A necessary course for anyone handling energy data – looking forward to implementing as much of the content as possible across our properties.
  • Highly recommend this course for energy professionals or anyone wanting to know to monitor their energy. Excellent trainer, thorough course with practical exercises – Kate Ingham
  • A very useful and insightful course which has helped me greatly with my work in energy analysis – Grace Sadler
  • The course was delivered in a relaxed and friendly manner where questions and queries were encouraged which greatly assisted in the understanding of the subject. I would recommend the course to anyone interested in setting up an energy monitoring and targeting scheme – Tim Howard
  • A fantastic course to increase knowledge and confidence – Vicki Rees
  • A really helpful course which has enabled me to understand monitoring and targeting further – Alistair Mann
  • Very convenient location, informal, pleny of opportunities to discuss particular aspects and areas of interest, Fairly simple approach and not over-complicated, allowing everyone to keep pace. Some great take-away technical information. Time well spent. The Vesma.com energy monitoring and targeting training is a must for people new to energy management or those with prior knowledge. Some great and simple techniques to understand and inprove your energy performace, backed up with real world experiences – Peter Bowman
  • I now understand CUSUM! I felt it was very good at providing an overview and the theory.
  • Format, pricing and contetnt were all very good. Vilnis added a lot of depth to my understanding. If you are genuinely interested in energy management and making data-informed decisions, Vilnis Vesma will put you on the right track. – DC
  • I think the penny has finally dropped! – Gwen Kinloch
  • A very straightforward yet clever way of making sense of mountains of data. – Ben Foster
  • A great introduction to the fundamental techniques of monitoring and targeting. Highly recommended. – Darren Holman
  • Was very helpful for building confidence. – NKC

Book onto the next available course at vesma.com/training

Additives in chillers: limits of plausibility

Most people who understand the physics of heating systems will understand why the claims for energy-saving boiler-water additives ring hollow. But similar products (possibly the same ones with different labels) are now being touted for central chiller systems and it is evident that some users have fallen for them. Time to look at these new claims.

First, as a quick potted revision of the context, I’ll describe an air-cooled chiller with ordinary basic control:

  1. Heat is abstracted from the building by means of a closed loop of chilled water leaving the chiller at a set temperature and returning at a higher, and variable, temperature.
  2. The water boils refrigerant in a heat exchanger called the ‘evaporator’ which operates at a temperature just below the chilled-water set point (the difference, called the approach temperature, is typically 2°C or less).
  3. After compression the refrigerant (now at elevated temperature) passes through an air-cooled heat exchanger called the ‘condenser’ which runs maybe 10-15°C hotter than ambient. Here latent heat is lost from the refrigerant, which condenses back to liquid.

Thermodynamically the key thing is the temperature ‘lift’ in the refrigerant between the evaporator and condenser. As users we are interested in the coefficient of performance (CoP) of the machine, which represents the ratio of cooling power out to electrical power in. The theoretical CoP is given by the formula:

Tc/(Th-Tc)

Where Tc and Th are the absolute refrigerant temperatures in the evaporator (cold) and condenser (hot). Let’s put some numbers on this as an example:

  1. Chilled water set point: say 6°C. This is self-evidently fixed.
  2. Evaporator approach temperature: let’s say 2°C. This is the thing which we might be able to influence by improving heat transfer.
  3. From (1) and (2) above we have an evaporator temperature (Tc) of 6-2=4°C or 277K
  4. Ambient air temperature: let’s go for 35°C
  5. Condenser approach temperature: let’s say 12°C
  6. From (4) and (5) we would have a condenser temperature (Th) of 35+12=47°C or 320K

So our theoretical CoP is

Tc/(Th-Tc) = 277/(320-277) = 6.44

(actual CoPs are always lower but that won’t matter if all we want is a comparison)

Now let’s repeat the calculation with an evaporator approach temperature of 1°C instead of 2°C. This means Tc will go up from 277 to 278K and our new CoP will be:

Tc/(Th-Tc) = 278/(320-278) = 6.62

This is slightly less than a 3% improvement,  but even that could well be an exaggeration because it assumes (a) that there was scope to reduce the approach temperature in the first place and (b) that poor heat transfer from the chilled water was the cause. Actually if the chilled water system is clean and properly treated it’s unlikely to be the problem: it’s a closed loop so contaminants won’t be getting in. In fact if the evaporator approach temperature is too high the reason is far more likely to be loss of refrigerant, or unbalanced distribution in the chiller, or oil in the circuit, none of which is affected by water treatment. It follows that if you suspect that your evaporator approach temperature is higher than it should be, look to ordinary maintenance steps first before anything else.

Vendors of additives will point to possible heat-transfer improvements within air-handling units or room air conditioning units. This is a red herring. These don’t affect the cooling load presented by the building, which is entirely and solely dictated by its heat gains and control settings.

Finally if anyone shows you a case study demonstrating an improvement, be skeptical. It’s far more likely they made the gains by cleaning the condenser coils, enabling evaporative cooling, servicing the chillers, or raising the chilled-water set point. These proven solutions are all things which perhaps are opportunities for you to exploit today.


Need training on energy management? Visit vesma.com/training

Frost protection: a point of weakness

Too often we see frost-protection thermostats set at too high a temperature, meaning that an unoccupied building will be heated for longer, and maintained at a higher temperature,  than is necessary for the purpose of preventing water services from freezing.

The diagram on the right shows how a common type of mechanical frost thermostat can be prevented from having too high a switching  temperature set. Or from being set dangerously low, for that matter. There is a protrusion on the dial and a series of bendable tabs is provided on a backplate which stop it at either end of the desired travel (in this case +2°C to +6°C). Your electrician will know how to do it.


Automatic control is one of the topics in our current season of energy technology briefings