Renewable Sexiness

I’m sure you’ve noticed that poor old Portugal hasn’t been having an easy time of late.  Granted, it’s not quite as down in the dumps as some of its European  neighbours, but being on the low end of the continuum of European woe and  misery isn’t exactly huge consolation.  However, and this is a however that counts, there is one thing Portugal can be proud of.  Actually, there are quite a few things, but let’s stick with one for the moment.  And no, it’s not football, but the arguably more honourable theme of renewable energy (hold your horses, Zé, I said “arguably” – no one’s disparaging your team!).

Portugal’s aggressive renewable energy policies have meant that it has already exceeded EU goals of 20% of consumed energy from renewable sources by 2020.  In fact, its increased goal for 2020 is 31% and it has beaten many of its more prosperous and less crisis-afflicted brothers in reducing its consumption of traditional fuels.

So what does that have to do with this blog?
Apart from significant investments in wind and even wave power, Portugal has set minimum solar requirements for new builds.  Anyone who is building a new house needs to put in a total of 1m2 for each person, so in a 3-bedroom house, you would be obligated to install 4m2, based on the assumption that there is one habitant per bedroom and 2 in the master bedroom.  This is a great step forward, but there is another, far sexier energy that is available and it is one we will be using in our new house.

Very Sexy and oh-so-Green

Since I’m no engineer, I’ll give you the simplified version and diagram of geothermal heating and cooling here, courtesy of the good people at Geothermal Genius.  In essence, geothermal (also known as geoexchange or ground source) heating and cooling are based on the fact that the earth’s surface temperature – the top 6 metres (20 feet) – is reasonably stable and hovers around 10-16 degrees Celsius (50-60° F).  This is generally warmer than the air temperature in winter and cooler than the air temperature in summer.
To heat or cool your house, a geothermal system makes use of our ability to exchange energy between the air and the energy taken from the ground.  To “harvest” the earth’s heat or coolness, a network of pipes or collectors is laid in an area close to the building anywhere between 1 and 2 metres (5-7 ft) deep if you’re using a horizontal system and typically between 80-100 metres (260-360ft) in the case of a vertical system.  These form a closed loop that is filled with water or glycol-water that acts as a conductor of the energy.  In winter, when the temperature in the house falls, heat from the earth is exchanged and pushed into the house with the aid of a heat pump.  In summer, the opposite happens: heat from the warm air in the house is exchanged for air that has been cooled down with energy from the earth.  In short, the earth acts as heat source in winter and a heat sink in summer.

It’s a pretty nifty system, but if you’re a reader who’s more into my more light-hearted posts about our ever-threatened sanity (okay, mine), you can happily go away until my next instalment about our latest challenge, mishap or obsession – I won’t reveal which it is at this point, but I promise you it’s one of them…

If you’re still with me, however, and have questions, like I had, here’s what our resident engineer – aka Tom the engineering whizz and man who drives Portuguese builders nutty – had to say:

J: Explain why direct geothermal heating is far more efficient than simple old electrical heating and why it has less demanding temperature requirements.

T:  Traditional heating converts some form of energy (electricity, natural gas, diesel, etc.), for which you have to pay, into heat.  During the conversion process, some of the energy that you pay for is lost due to inefficiencies of the mechanical systems that perform the conversion (i.e. the consumer pays for the actual heat produced plus all the mechanical inefficiencies in the system).  Geothermal heating basically transfers heat that resides in the earth’s upper surface to your house using a heat pump.  The energy for which the consumer pays is only that used in the mechanical systems that move heat from one place to another (a heat pump and circulation system).  Based on current commercially available technology, using geothermal heating gives you 4-5 units of heat for every unit of energy used to mechanically transfer that heat (i.e. you pay for one unit, and the system delivers 4-5 units to your house).  One might then ask, “Where does the heat that resides in the earth’s surface come from?”  Fortunately for us, the heat that resides in the earth’s upper surface is replaced by a combination of solar radiation from the sun and the heat from the molten core of the earth that slowly migrates to the surface.

J: Is it prohibitively expensive?  Or why is this technology, that’s been around for decades, so unadopted?

T:  Regarding cost, the upfront capital cost is significant (upwards of EUR8000), but usually has a payback period of 7 to 10 years.  This is highly influenced by the cost of electricity, other alternative energy sources, and any tax breaks that may be available. 

Regarding the “why now”, as with all things, necessity is the mother of invention, or possibly that should be modified to “perceived necessity”.  Until the price of electricity and fossil fuels increased enough to make people complain, no one really was driven to pursue technologies that required significant development for commercialisation (solar, wind, etc.).  Or from another perspective, the engineers had not been pestered enough to move their backsides.  To more fully describe what motivates an engineer to do things, one would have need to carry out endless research, calculations, etc, etc, etc.

J:  Don’t I know it!  How big an area do you need for the collectors?  Can this system work on a small plot of land?

T:  Now we arrive at engineer speak.  First, it depends on the type of system you are going to bury in the earth to collect heat/coolness, so I will stick to horizontal collector systems.  The rules of thumb that are given in most commonly found information sources range from 1.25 to 1.60 square meters of land per square meter of indoor living area.  However, the actual number will depend on soil type, moisture level, depth of pipes to be buried, which all influence the rate at which heat is conducted from the earth to the collection system.  The area should be free of buildings, paving, and other obstacles that may reduce transfer of energy in the collection area.

J: As far as I understand there are several potential technologies involved and many variations of the basic system.  We wanted to use a German provider that has since scaled down its operations in Portugal – a sad and unsexy symptom of the crisis – but we’ve since changed supplier and also equipment to something you didn’t initially favour.  What options are there and how on earth (no pun intended) does one decide what to go for?

T:  The fun part (from an engineer’s perspective) of these heating/cooling systems is that they are forever developing and changing.  Because geothermal/ground-source technology is still in a rapidly developing stage, commercially available systems change quite quickly (not iPhone quick, but every couple of years which can have a large impact on the economics of a house built for a 40-50 year life span).   The only sensible approach seems to be to speak with several (3 or more) suppliers and use their expertise to narrow down the possible solutions for your given situation.  Once you decide on a particular configuration, make sure the supplier can take you to an existing similar installation and can demonstrate to you that the system has been working for 2 or more years.  This is important because these systems fluctuate with the different seasons of the year, and some take 3-4 years to reach steady state.  Also, make sure the supplier will be able to reliably provide the initial set-up and annual servicing thereafter.

If you really want a challenge, let your engineering spouse come up with a hybrid system…

J:  Please, no more challenges!  Can it work in all climates?

T:  There seem to be working geothermal systems in all climates.  Horizontal collector systems reportedly work in all climates other than those where permafrost extends to a level deeper than the level at which collection pipes are buried – typically 1-2 metres deep.

J:  What if you have a bitterly cold day and the heat harvested from the earth isn’t sufficient?  You know what my toes are like…

T:  These systems are usually designed to work on the coldest (and warmest) day of the year – a good question to ask potential suppliers.  However, given the apparent increase in general weather pattern extremes, it helps to have a back up.  In our case, it is an efficient fireplace during the winter and a swimming pool for the summer.  Some suppliers combine the use of solar panels for heating, although the systems can become quite technically challenging due to different operating temperatures, the variability of cloud cover, and the connecting pipework.

J: Apart from tree roots that could upset the whole system, what other risks of damaging the systems are there?

T:  Any major earthwork near the pipes or heavy equipment movement on the area of the horizontal collector could damage the system.  Some of the commercially available systems reportedly are not susceptible to tree root damage.

J:  Are there any obvious keys to success like opting for German technology?  Any potholes of disaster you’ve discovered?

T:  1. Never fully trust what an engineer says – they are very good at coming up with reasons why something did or did not happen…

J:  Did you really just say that?  You realise I might have to quote you on this one day…

 T:  …2. Speak with several suppliers and parallel process their offers up until you have a firm idea of the total cost from each.  3. Do the research in terms of customer reviews of the company, the service they supply after the system is installed, and the heat pump manufacturer.  Still on the lookout for “potholes”, will inform once we are deeply embedding in one…

J:  Thanks for this, senhor engineer.  Not only am I better informed about geothermal energy, but I have a number of confessions about the true nature of engineers in writing and I feel rather good about future points of eh, shall we say “discussion”?  We should do this more often.

Learn more about geothermal heating and cooling and the cost of geothermal heat pumps – visit


4 responses to “Renewable Sexiness

  1. Love it! And can just imagine this ‘interview’ taking place:)) xx

  2. Nice Job! I like the way you explain the efficiency and how it all
    I will be posting this article at my geothermal

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