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Fuel savings from system water treatment: limits of plausibility

Just how big a saving is it possible to achieve with a product which improves heat transfer in a ‘wet’ heating system (one which uses circulating water to feed radiators, heater batteries or convectors)?  It is an important question to answer because suspect additives claiming to reduce losses through water treatment are becoming prevalent, making claims in the range of 10-20%, while air-removal devices have been claiming up to 30%. It is possible to show that the plausible upper limit is of the order of 7%  and that this would be achievable through good routine maintenance anyway.

To work this out we first break the system into its two major components: the heating boiler (which in reality may be two or more plumbed in parallel) and the building, which represents the heat load. The first thing we can say is that if the heating in the building is maintaining the required temperatures, the thermal load which it presents to the boiler will not be affected by internal heat transfer coefficients. If heat transfer in the heat emitters is impeded, then either the circulating water temperature will rise or control valves will be open for a greater percentage of time in order to deliver the required heat output, or both; either way, the net heat delivered (and demanded from the boiler) is the same.  So water treatments will not affect the heat demanded from the boiler; their only effect will be to improve the efficiency with which the boiler converts fuel into useful heat.  Let us consider how this can be done. Consider the routes by which energy is lost in the boiler:

  1. Standing losses from the boiler casing and associated pipework and fittings;
  2. Sensible heat loss in the exhaust gases. This is the energy that was needed to elevate the temperature of the dry products of combustion (i.e. excluding latent heat);
  3. Latent heat losses, e. the energy implicitly used in converting water to vapour in the exhaust (it is this heat which is recovered in a condensing boiler);
  4. Unburned fuel (carbon monoxide or soot).

Which of these could be affected by water treatment and which would not?  Standing heat loss is sensitive only to the extent that the external surface temperature of the boiler might differ with and without water-side scaling. As such losses would only be about 2% of the boiler’s rated output in the first place, we can safely take the effect of variations to be negligible. Latent heat losses would not be affected because they are solely a function of the quantity of water vapour in the exhaust, and that is fixed by the chemistry of combustion and in particular the amount of hydrogen in the fuel. Unburned fuel losses will not be affected either. They are determined by the effectiveness of burner maintenance in terms of air/fuel ratio and how well the fuel is mixed with the combustion air.

That just leaves sensible heat losses.  Two things can cause higher-than necessary sensible heat loss. One is to have excessive volumes of air fed through the combustion process, and the other is having a higher-than-necessary exhaust gas temperature.  Excess air is self-evidently totally unrelated to poor water-side heat transfer, but high exhaust temperatures will definitely occur if the heat transfer surfaces are dirty or scaled up.  With impaired heat transfer the boiler cannot absorb as much of the heat of combustion as it should, or to look at it a different way, higher combustion-product temperatures are needed to overcome the thermal resistance.

Elevated stack temperature, then, is the only significant symptom of water-side scaling.  So how high could that temperature go, and what are the implications?  Most people would agree that an exhaust temperature of 250°C or more would be highly exceptional and values of 130°C to 200°C more typical.  Now let us suppose for the sake of argument that the exhaust gases in a reasonably well-maintained boiler contain 4% residual oxygen in the exhaust and have a temperature of 130°C, with (to make it realistic) 200 parts per million of carbon monoxide. The stack losses under these conditions will be:

4.2% sensible heat in dry flue gases

11.2% enthalpy of water vapour

0.1% unburned gases.

This leaves a net 84.5% as “useful” heat but we should deduct a further 2% for standing losses, giving 82.5% overall thermal efficiency as our benchmark.

Now let’s suppose that the same boiler had badly fouled heat transfer surfaces, raising the exhaust temperature to 300°C —  way in excess of what one might normally expect to encounter.  Under these conditions the stack losses become:

10.4% sensible heat in dry flue gas

12.7% enthalpy of water vapour

0.1% unburned gases

So we now have only 76.9% “useful” heat which, after again deducting 2% standing losses, means an overall efficiency of 74.9%, compared with the 82.5% benchmark.  The difference in efficiency between the dirty and clean conditions is

(82.5 – 74.9) / 82.5 = 6.8%

and this figure of about 7% is the most, therefore, that one could plausibly claim as the effect of descaling a heating system whose boilers are otherwise clean and reasonably well-tuned. In fact if the observed stack temperature before treatment is lower, the headroom for savings is lower too.  At 200°C the overall efficiency would work out at 81.4% and the potential savings would be capped at about 3%.

Three points need to be stressed here. Firstly, just measuring the flue gas temperature will tell you accurately the maximum that a boiler-water additive alone could conceivably save. Secondly, you cannot be sure the problem is on the water side anyway: it may be fireside deposits. Thirdly, all these potential savings should be achievable just with good conventional cleaning and descaling.

 

Attitudes to energy: a radical survey approach

Six years or so ago I was asked to help with an energy awareness and motivation campaign at a major conference and banqueting venue. One of the elements I was responsible for was the initial attitude survey, and I decided to approach it in a slightly unusual way, inspired by two textbooks* that I use in training workshops.

There were a couple of psychological phenomena that I wanted to exploit. One was ‘social proof’, the tendency of individuals to act in a way that they think other people like them would act in the same circumstances; the other was the power of informal friendship groups, which tend to bind people more closely than any formal organisational relationships. Also, given that I was dealing with waiters, porters, cleaners, cooks and security guards, I knew from experience that an on-line survey (fashionable at the time) was not the way to go because many of them would not have been able or willing to respond that way. It had to be paper.

Furthermore I wanted to get away from multiple-choice questions. We all know that the reply we would choose is never offered, and I was smarting from an an earlier staff survey for the Environment Agency in Wales, in which people had bombarded the free-text comment boxes with valuable thoughts. Lots and lots of valuable thoughts. So I did two things. I made the questionnaire one page, with just four open-ended questions, and I asked people to talk through the questions with their friends and come up with group responses if they could (otherwise to report dissenting views). The four questions I asked were:

  • Do you think there is significant energy waste at XXX? If so, what and where, and whose job should it be to reduce it?
  • What other aspects of work are more important than saving energy?
  • If you think energy saving is important, why?
  • Does anyone in the group feel they would benefit from special training to help them work in a more energy-efficient manner? What would they like to know more about?

Normally for an organisation with hundreds of staff you would never do this; you would go mad analysing the replies. But with group responses, you have numerically only a fraction of the material to sift through. You are also getting people to discuss the matter in hand, which in itself starts them on the path to engagement with the subject.

The results in this case were telling. Firstly, the vast majority of replies to the question whose job it should be to reduce energy waste said it was everyone’s. Even more striking was that every reply identified cost as a thing that makes energy important (just over half additionally mentioned the environment). Some suggested that the savings could be spent on bringing in more business, and thereby securing long-term employment. And when it came to what was more important than saving energy, the overwhelming majority said customer service. Not in a million years would I have thought of making ‘customer service’ one of the possible responses in a multiple-choice question but that was most groups mentioned. I’d like to quote one response in particular:

“Customers must receive a professional, efficient friendly service carried out by conscientious, smart, knowledgeable staff, who show pride in their working environment, resulting in customers returning again ”

So there we have it: waiters, porters, cleaners, cooks and security guards thinking like owners and managers. Furthermore, almost everyone believed there was energy waste at work (the only exception, tellingly, came in an individual response from a director). Not surprisingly lighting was seen as the main culprit, though other things got one or two mentions like the behaviour of event set-up crews.

On the strength of the consensus in the replies, I circulated a single-page summary back to all staff. I have no idea which of them had participated in the survey; the important thing was for everyone to see that their colleagues tended to share a common view which, from the overall project perspective, was positive. Social proof – their instinct to conform to perceived norms – would help us to the next step.


* the two textbooks that I recommend my students to read before workshops on motivation are: “Yes: 50 Secrets from the Science of Persuasion” by Goldstein, Martin and Cialdini, which is still in print; and “The Social Psychology of Industry” by J.A.C.Brown. First published in 1954 and now out of print, this is a difficult read which occasionally challenges our modern sensibilities, but it repays the effort.

Pitfalls of regression analysis: case study

I began monitoring this external lighting circuit at a retail park in the autumn of 2016. It seems from the scatter diagram below that it exhibits weekly consumption which is well-correlated with changing daylight availability expressed as effective hours of darkness per week.

The only anomaly is the implied negative intercept, which I will return to later; when you view actual against expected consumption, as below, the relationship seems perfectly rational:

 

Consumption follows the annual sinusoidal profile that you might expect.

But what about that negative intercept? The model appears to predict close to zero consumption in the summer weeks, when there would still be roughly six hours a night of darkness. One explanation could be that the lights are actually habitually turned off in the middle of the night for six hours when there is no activity. That is entirely plausible, and it is a regime that does apply in some places, but not here. For evidence see the ‘heatmap’ view of half-hourly consumption from September to mid November:

 

As you can see, lighting is only off during hours of daylight; note by the way how the duration of daylight gradually diminishes as winter draws on. But the other very clear feature is the difference before and after 26 October when the overnight power level abruptly increased. When I questioned that change, the explanation was rather simple: they had turned on the Christmas lights (you can even see they tested them mid-morning as well on the day of the turn-on).

So that means we must disregard that week and subsequent ones when setting our target for basic external lighting consumption. This puts a different complexion on our regression analysis. If we use only the first four weeks’ data we get the relationship shown with a red line:

In this modified version, the negative intercept is much less marked and the data-points at the top right-hand end of the scatter are anomalous because they include Christmas lighting. There are, in effect, two behaviours here.

The critical lesson we must draw is that regression analysis is just a statistical guess at what is happening: you must moderate the analysis by taking into account any engineering insights that you may have about the case you are analysing

 

Lego shows why built form affects energy performance

Just to illustrate why building energy performance indicators can’t really be expected to work. Here we have four buildings with identical volumes and floor areas (same set of Lego blocks) but just look at the different amount of external wall, roof and ground-floor perimeter – even exposed soffit in two of them.

But all is not lost: there are techniques we can use to benchmark dissimilar buildings, in some cases leveraging submeters and automatic meter reading, but also using good old-fashioned whole-building weekly manual meter readings if that’s all we have. Join me for my lunchtime lecture on 23 February to find out more

Advanced benchmarking of building heating systems

The traditional way to compare buildings’ fuel consumptions is to use annual kWh per square metre. When they are in the same city, evaluated over the same interval, and just being compared with each other, there is no need for any normalisation. So it was with “Office S” and “Office T” which I recently evaluated. I found that Office S uses 65 kWh per square metre and Office T nearly double that. Part of the difference is that Office T is an older building; and it is open all day Saturday and Sunday morning, not just five days a week. But desktop analysis of consumption patterns showed that Office T also has considerable scope to reduce its demand through improved control settings.

Two techniques were used for the comparison. The first is to look at the relationship between weekly gas consumption and the weather (expressed as heating degree days).

The chart on the right shows the characteristic for Office S. Although not a perfect correlation, it exhibits a rational relationship.

Office T, by contrast, has a quite anomalous relationship which actually looked like two different behaviours, one high one during the heating season and another in milder weather.

The difference in the way the two heating systems behave can be seen by examining their half-hourly consumption patterns. These are shown below using ‘heat map’ visualisations for the period 3 September to 10 November, i.e., spanning the transition from summer to winter weather. In an energy heatmap each vertical stripe is one day, midnight to midnight GMT from top to bottom and each cell represents half an hour. First Office S. You can see its daytime load progressively becoming heavier as the heating season progresses:

Compare Office T, below. It has some low background consumption (for hot water) but note how, after its heating system is brought into service at about 09:00 on 3 October, it abruptly starts using fuel at similar levels every day:

Office T displays classic signs of mild-weather overheating, symptomatic of faulty heating control. It was no surprise to find that its heating system uses radiators with weather compensation and no local thermostatic control. In all likelihood the compensation slope has been set too shallow – a common and easily-rectified failing.

By the way, although it does not represent major energy waste, note how the hot water system evidently comes on at 3 in the morning and runs until after midnight seven days a week.

This case history showcases two of the advanced benchmarking techniques that will be covered in my lunchtime lecture in Birmingham on 23 February 2017 (click here for more details).

Air-compressor benchmarking

Readers with reliably-metered compressed-air installations are invited to participate in an exercise using a comparison technique called parametric benchmarking.

Background

Traditionally, air-compressor installations have been benchmarked against each other by comparing their simple specific energy ratios (SER) expressed typically as kWh per normal cubic metre. However, as this daily data kindly supplied by a reader shows, there may be an element of fixed consumption which confounds the analysis because the SER will be misleadingly higher at low output:

example
Note: a four-day period of anomalous performance has been hidden in this diagram

It seems to me that the gradient of the regression line would be a much better parameter for comparison; broadly speaking, on a simple thermodynamic view, one would expect similar gradients for compressors with the same output pressure, and differences would imply differences in the efficiency of compression. The intercept on the other hand is a function of many other factors. It may include parasitic loads; it will certainly depend on the size of the installation, which the gradient should not.

I am proposing to run a pilot exercise pooling anonymous data from readers of the Energy Management Register to try “parametric” benchmarking, in which the intercepts and gradients of regression lines are compared separately.

Call for data

Participants must have reliable data for electricity consumption and air output at either daily or weekly intervals: we will also need to know what compressor technology they use, the capacity of each compressor, and the air delivery pressures.

In terms of the metered data the ideal would be to have an electricity and air meter associated with each individual compressor. However, metering arrangements may force us to group compressors together, the aim being to create the smallest possible block model whose electricity input and air output is measurable.

Please register your interest by email to moc.a1490256670msev@1490256670sinli1490256670v1490256670 with ‘compressor benchmarking’ in the subject line: once I have a reasonable group of participants I will approach them for the data.

Vilnis Vesma

4 January 2017

Meaningless claims

MEANINGLESS CLAIMS No. 9,461

Seen in a product brochure for a control system: “The theory states that if you allow the indoor temp to vary by 8ºC in a commercial or public building the heat saving will be 80%. In practice a span of 3-4ºC is usually more realistic (20-24ºC is common) resulting in heat savings of 20-40%. The use of a temperature range does not mean that the indoor temperature will change 3-4ºC over 24h, the average change in indoor temp over 24h is less than 1ºC, which is enough to utilise thermal storage. If no range is allowed, none of the excess free or purchased energy can be stored in the building.”

MEANINGLESS CLAIMS No. 9,462

I recently reported the new fashion for describing boiler-water additives as ‘organic’ to make them sound benign. As I pointed out, cyanide is an organic compound. Now here’s a new twist: a report on the efficacy of a certain boiler water additive says “[it] is 100% organic so the embodied carbon is 0.58kg of CO2 per bottle”. Er… How do they figure that?

MEANINGLESS CLAIMS No. 9,463

The same report cited another which said that a certain programme of domestic energy-conservation refits had yielded “up to a 42% increase in living room temperature”. Cold comfort indeed if your room started at zero degrees Celsius; 42% of zero is zero. Oh wait: what if you had used Fahrenheit, where freezing point is 32°F? A 42% increase on 32°F gives you 45.4°F (7.5°C). So it depends what temperature scale you use, and the truth is you can only talk about a percentage increase in temperature relative to absolute zero (-273°C). If we start at an absolute 273K (0°C), a 42% increase takes us to 388K or 115°C. To be honest, that doesn’t sound too comfortable either.

Refrigeration nonsense

The vapour-compression cycle at the heart of most air-conditioning systems consists of a closed loop of volatile fluid. In the diagram below the  fluid in vapour form at (1) is compressed, which raises its temperature (2), after which it passes through a heat exchanger (the “condenser”) where it is cooled by water or ambient air. At (3) it reaches its dewpoint temperature and condenses, changing back to liquid (4). The liquid passes through an expansion valve. The abrupt drop in pressure causes a drop of temperature as some of the fluid turns to vapour: the resulting cold liquid/vapour mixture passes through a heat exchanger (the “evaporator”) picking up heat from the space and turning back to vapour (1).

normal_loop
Figure 1: the vapour-compression refrigeration cycle schematically and on a temperature-entropy diagram

The condenser has two jobs to do. It needs to dump latent heat (3->4) but first it must dump sensible heat just to reduce the vapour’s temperature to its dewpoint. This is referred to as removing superheat.

It has been claimed that it is possible to improve the efficiency of this process by injecting heat between the compressor and condenser (for example by using a solar panel). Could this work?

solar_loop_true
Figure 2: showing the effect of injecting heat

The claim is based on the idea that injecting heat reduces the power drawn by the compressor. It is an interesting claim because it contains a grain of truth, but there is a catch: the drop in power would be inextricably linked to a drop in the cooling capacity of the apparatus. This is because we have now superheated the vapour even more than before, so the condenser now needs to dump more sensible heat. This reduces its capacity to dump latent heat. The evaporator can only absorb as much latent heat as the condenser can reject: if the latter is reduced, so is the former. Any observed reduction in compressor power is the consequence of the cooling capacity being constrained.

The final nail in the coffin of this idea is that reduced power is not the same as reduced energy consumption: the compressor will need to run for longer to pump out the same amount of heat. Thus there is no kWh saving, whatever the testimonials may say.

View a vendor’s response

Fifty years of degree-day data

Another exhibit for the Museum of Energy Management: thanks to David Bridger for unearthing UK monthly degree-day data for the period 1966 to 1975 (view the complete archive file here ).

These data have mainly curiosity value and should not be relied upon for any kind of trend analysis. Observing stations have sometimes been moved or affected by nearby building development and urban expansion. Region 18 (North West Scotland) was not even included at all until I launched the Degree Days Direct subscription service in 1992, and there had been two other main data providers before I got the contract to supply the official figures in 2003, so it would be risky to assume continuity over the whole fifty years.

Below: degree-day figures reported in the government’s “Energy Management” newspaper in 1986

old_dd_table

Effect of voltage on motor efficiency

Proponents of voltage reduction (“optimisation” as they like to call it) have started suggesting that equipment is more energy-efficient at lower voltage. In fact this is quite often not the case. For an electric motor, this diagram shows how various aspects of energy performance vary as you deviate from a its nominal voltage. The red line shows that peak efficiency occurs, if anything, at slightly above rated voltage.

motor_voltage_characteristics

Reduced voltage is associated with reduced efficiency. The reason is that to deliver the same output at lower voltage, the motor will need to draw a higher current, and that increases its resistive losses.