Summer finally has arrived in Switzerland. Yes, I live in a moderate climate zone, but if the outside temperature goes above 28-30° Celsius as these days, then sleeping at night is not that comfortable as it should be in my view. Luckily, I’m in a good constructed house with good insulation, so it takes a few days until it heats up. But I love to keep the temperature below 25° Celsius, especially at night. I do have a heating system which combines geothermal and solar heating. The question is: how can I use it for cooling during hot summer days? The solution: some extra plumbing, a Freescale Tower system and the Freescale FRDM-KL25Z board
Warning and Disclaimer! Serious mechanical, electrical, electronic and software engineering skills are needed if you want to do something like this. Be aware that the thermal and electrical power used in this project can dangerous. Do it at your own risk! And there is no guarantee what worked for me will work for you. I conducted this project with the professional tips of my colleagues at the University of Lucerne for Applied Sciences and Arts (Thermal Energy Systems and Process Engineering), and if you do not have the expertise or knowledge, you better do not start such a project ;-).
Ending the Oil Burning Area
Last year, my old heating system based on burning oil started to need increased maintenance. The same time the oil prices were going higher and higher. So the idea was to replace the old system and replace it with a more CO2 neutral solution, especially as the government in Switzerland offers tax and funding incentives. So we decided to get rid of the oil tanks and oil burning system, and replace it with a geothermal plus solar heating system. Of course such a project needs a lot of administrative work to get all the required permissions (environment safety, construction regulations, etc). But finally I had all the paperwork done, it was time to actually get some work done :-)….
The first phase was to drill a 186 meter hole into the ground. Geothermal drilling and heating are getting a standard. But to me that drilling machine was still impressive, and I was thinking that maybe in my next life I should be a mechanical engineer ;-):
The drilling machine needs a big compressor for high pressure water supply:
As the underground was special and the area is on an old landslide area, the first 50 meters required special pipes to avoid the drill hole to collapse. So outside of the drilling head another pipe with larger diameter gets installed:
The water is pressed into the drilling hole both to cool the drilling head plus to get drilling material out of the hole:
Adding a new pipe is fast and efficient, so the drilling took only two days:
The water with the drilled material gets collected in a container:
Regularly, the container (two were installed in parallel) needed to be drained by a big pump truck:
But around 23 meters, the drill master was swearing about something strange was going on. And indeed, in the drill water container something strange showed up: pieces of wood!
So this confirmed that we are on an old land slide area: we hit with the drill machine a more than 7000 year old cedar tree!
Goldauer Rock Slide from 1806
That tree got buried in a pre-historical rock and landslide after the ice age. When the glaciers retrieved after the last ice age it has caused several land and rock slides in the area. The most recent is from 1806 which buried the Goldau village nearby:
Every 20-50 meter there is a layer of marl which gets slippery if in contact with water. This layering is clearly visible in the top rock slide area. And the rainy and wet summer of 1806 caused that big rock slide, and killed 457 people (see Goldauer Bergsturz):
The area still has not stabilized yet, as shown by a smaller landslide event back in November 2005:
So I keep fingers crossed that it keeps stable for the next few hundred years ;-).
Back to drilling :-).Because that area is of interest for any geologists, I had to collect a sample every meter of drilling. Based on this, a detailed underground profile is the result:
To bad: no treasures found ;-). And drilling was down to 180 meters.
Back-Filling the Drilling Hole
To get the geothermal heat, a special fluid will be circulating in the hole. For this, two U-Shaped pipes (4 single pipes in total) get installed:
Two such U-shaped pipes get combined with a heavy metal piece at the bottom:
Tied together, the pipes are ready to be put down:
With the help of the metal weight, the weight of water filled in the pipes and air pressure everything was pulled down with minor issues:
An extra pipe is added to the two U-Pipes, it is used later for backfilling the hole from the bottom with mortar:
Pushing things down worked for the first 52 meters. Then things were stuck:
Because the drilling hole was filled with water, this created enough up-pressure that the pipe did not go down further. To counter this, the pipes were flushed and filled with water. With a lot of water to get any air out of the system. This increased the weight of the tubes to get them down 100 more meters:
Still 30 meters to go! But things are stuck again. The problem is the pressure down at 150 meters, plus the friction of the pipes in the hole. What remains is a rather risky procedure: to blow pressurized air into the backfilling pipe to get the water out of the bottom of the hole. This would remove the counter pressure from the bottom, but the same time the risk is that due the reduced pressure it is possible that the hole will collapse :-(. But there is not much else you can do, so we agreed on taking the risk. With the compressor pressurized air is put into the hole, pushing the water out of the drilling hole:
With this, we made the last 30 meters without issues :-).
At the end, this is what remains on top is this:
The drilling hole gets backfilled with mortar: for this, a 5th tube has been put down so the hole can be filled from the bottom up. The mortar gets mixed with water and pumped down:
Instead of using the normal mortar to fill the drilling hole, I used a special one with better heat transmission value (2.32 W/(m-K)) instead of the normal 1 W/(m-K):
Higher W/(m-K) eases the energy flow from/to the geothermal probe with the surrounding rock.
At the end, everything gets put into the ground and into the building. Both U-shaped pipes get combined into a larger one:
The pipes end up at the head pump which stores the energy inside a water buffer:
In addition, 12 thermal solar panels supply energy to the system:
The solar panels are about 30 m2 and can give enough energy during summer time (actually too much). And they help the heating system during winter time. If the sun shines on a winter day, I get enough energy for 2-3 days. It was not possible to put the panels on the house roof: the house roof is East-West oriented, so not possible to place the panels facing south. Plus the angle would have been too flat: at wintertime the sun is pass low on the sky, so having the panels more vertically oriented with an angle of 70-80° is ideal for such a heating-aid system.
So far this is all about heating. So where is the cooling part? The basic idea is to use the underground as an extended buffer to store the not needed heat during summer time. So the geothermal probe is used to get energy during winter time, and to store energy during summer time. There are two sources of exceed heat which can be used:
- Solar Energy: During a sunny summer day, the thermal solar panels produce up to 90 kWh a day, far mor than it is consumed. If it would be sunny during several days, the system would stagnate because of overheating. While the system can deal with stagnation, it is not good in general because of the high temperature in side the solar panels. So the idea is instead the system to allow to go into stagnation, using a predictable controller which pumps the extra energy into the ground.
- Cooling the building: In hot summer time, the building is heating up above 25° C, while the geothermal probe temperature is around 12° C. I have a low temperature floor heating system, so the idea is to make a ‘free cooling’ system where the fluid in the floor heating system is cooled by the lower temperature of the ground.
In addition, I’m using a small outdoor pool with the extra energy available during summer time.
Everything is still somewhat experimental, but to deal with the above two cases, I installed
- Cooling Heat Exchanger between the buffer/floor system and the geothermal flow.
- Floor Cooling Mixing Valve to control the floor cooling temperature. I need to make sure that the temperature does not go into the condensation state.
- Buffer Cooling Mixing Valve to limit the temperature going into the geothermal/cooling heat exchanger from the buffer. I’m using PE as geothermal pipe, and although PE is stable up to 60-80° C, I want to limit the heat going into the pipe.
- Cooling/Heating Switch which turns either cooling or heating on. It has an extra helping relays which is used by the controller.
The schematics below shows the hydraulic system:
I measure temperature and pressure with Grundfos Digital Direct Sensors:
Sensors and actuators are managed by a control system based on Freescale Tower Modules, more about this later. As backup (I did not expect my control system to work right away a normal Resol solar controller is used).
Hacking the Hoval Heat Pump
The problem is that by default the Hoval heat pump used is not ready for such a cooling functionality, and the internal controller is not able to easily handle this. After going back and forward on different solutions, I decided to hack the pump electronics for my needs. All what I need is to make sure that the pump is not turning on the compressor, plus I need to turn on the circulation pump of the geothermal system.
For this I internally switch off the heating (and compressor system), and I’m misusing an internal maintenance mode which is used to flush the geothermal system. This mode is accessible using the ‘Manual’ mode button on the control panel:
To control that button plus the pump, I’m going to modify the heat pump system and adding a FRDM-KL25Z to do all the needed internal controls.
First, to open the heat pump system to get access to the backside of the control panel:
The first thing is: I need to rewire the pump and valves from the heat pump to my control system: for this I have them detached from the Hoval control board and wired it to my control system. It looks messier than it is :-):
To control the Hoval heat pump from my control system, I had to detach the panel and to remove the back cover:
Then need to identify the push button on the front side of the panel:
Two wires get soldered to the ‘manual’ push button:
The wires get to the backside of the panel:
The wires plus the wires for the pump are attached to a relay print. I’m using a dual channel opto-isolated relay print from YourDuino. As 230V AC are involved, everything is put into a plastic protective box and strapped to the backside of the panel:
On the other side there is a FRDM-KL25Z with an ARM Cortex-M0+. As on this side of the panel everything is low voltage, the board is directly attached to the back side of the panel:
The FRDM-KL25Z is connected through USB CDC (see this post) to my other controller. The Arduino Data Logger Shield is used for logging on SD-Card. The Maxim RTC on the shield provides time stamping and stand-alone operation of the unit: that’s why there is an on-off switch attached to the shield which is accessible from the front panel. As file system the FatFs component is used, and the board runs FreeRTOS tasks using CodeWarrior for MCU10.4.
With this the system performs automatic cooling as needed or required. And with the manual switch I can easily overwrite the controller:
Actually, that external ‘manual switch’ was a requirement of my wife: she wanted an easy and accessible way to overwrite the system behaviour. Maybe she did not trust my engineering skills? Or simply she did not want to control the system with the web browser? Anyway, I’m convinced that this manual switch is useful, even if I do not plan to use it for myself
Control System: VBus Gateway and Solar System Controller
The overall control system is built with Tower Modules using a ColdFire MCF52259. It is based on the work of Markus Bättig who developed a VBus Gateway for the Grundfos Direct Sensors which measure flow, temperature and pressure in the system:
The other part is the control software implemented by Marc Barmettler: it provides a graphical front end using the TWR-LCD module. Everything is based on Processor Expert graphical components:
Additional details on these two subsystems would be worth other blog posts. Post a comment if you want to know more about it :-).
Geothermal Probe Regeneration and Cooling
Let’s see how this works out: below is a snapshot of cooling the solar buffer and putting the energy into the underground: From the buffer I get 45.8° C. The thermal probe has been regenerated from the starting temperature of 12.5°C to 16.4°C, and I’m heating it up with a delta-T of about 5°C: typically I can pull down around 10-15 kW :-).
There is ‘live’ status of the system available here. Still experimental, and the ftp upload does not work all the time.
From the solar system I have a peak energy flow of 30 kW. But as I can use the night-time with lower energy costs, I easily can get the energy down into the geothermal system. With an investment of 50 Watts for controller an pumps, I can pump 10 kW of energy into the ground :-). Because of lower temperature, the floor cooling is not as good as this, but still 1.5 kW. But this is not that big of a problem: the floors are very inert, and it is know in advance if there is a heat wave coming. So I can cool slowly down in the nights before, getting me a comfortable house temperature.
The system is still experimental, but works very well so far. I’m able to cool about 1.5 kW from the floor system, and about 10 kW from the buffer. The pumps and electronics need around 50 W, so not bad compared to a traditional air conditioning system :-). Additionally, I can regenerate the geothermal system during summer time to get a better delta temperature during winter time, which should improve the overall system performance. In retrospect, I should have selected a larger heat exchanger, just to get energy transmitted more efficiently. What surprised me is the heat pump vendors do not consider to use the geothermal system for cooling. It exists for industrial systems, but somehow they do not realize that with little effort and investment the comfort and usability of a system can be greatly enhanced. Especially a joint geothermal+solar system sounds ideal to me for such an application. To make it work, I had to build my controller and heat pump hack based on the FRDM-KL25Z. Another ‘cool’ usage of the Freedom board in my view :mrgreen:.