Resource Cool - Building Instructions
Proposed air conditioning and heating system for use in low resource settings, where grid independency is desired or as an alternative to large battery banks.
1. Introduction
Accounting for seasonal and regional climate, heating/cooling can consume as much as 70% of the total energy usage of a building. In off-grid setups, this can require large battery banks or generators to continue heating/cooling when your renewable energy is not producing. For on-grid solutions, this can mean high energy bills for winter and summer months.
While working for an NGO on the west coast of Africa in 2021 I was tasked with finding an air conditioning solution for some rented buildings. As there were many downsides to the typical setup of diesel generators and temperamental A/C units, I decided to try something different and set myself the following goals.
Criteria:
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is solar powered
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can continue running after sunset without the need for a large battery bank
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is as locally sourced as possible
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uses low tech equipment
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uses readily available equipment and parts, that can easily be serviced by most technicians
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uses less refrigerant gas
In the end, I created an inexpensive system that achieved the above criteria, which was used in the buildings of the NGO. This system was then repeated in Australia but with the addition of wintertime heating. I believe both systems are attractive for people living off grid, people in low resource regions or people looking to cut their energy bill.
There are many areas of the world where this system could have an impact on energy consumption. In its simplest configuration, most of the work can be done by someone with an ok level of technical knowledge. Having access to scrap equipment is always a plus and can make things more fun as well. However, building a system like this requires working with both AC and DC voltages at a level high enough to be dangerous. Make sure proper knowledge is acquired and precautions taken before working with these connections and cabling.
I have tried to keep this document as short as possible while making it readable for a wide audience. The document is not a step-by-step instruction for duplicating the system I built; it is more a guide to assist you building your own. My hope is to inspire others to think outside the box and look at cheaper and more environmentally friendly solutions to heating and cooling, in addition to inspire people to utilize scrap they can easily find.
2. Concept
The terms “heat-pump” and “air conditioner” are commonly used when referring to compressor-based heating and cooling. In this document I have chosen to mostly use the term heat-pump.
The basics of a heat pump require a hot side and a cold side - there needs to be something that gets heated up if something is cooled down. Quite often only one side of this system is of interest at a time, the other side being either waste heat or waste cold. This waste thermal energy is then dumped into the outside air using radiators and fans. The waste thermal energy can also be dumped into the ground using pipes or into a body of water.
Of the two systems I have built, the first was used only for cooling with waste heat being dumped into the outside air. The second system was used for both heating and cooling the building. Because both systems are using solar as the power source, the location of installation needs to have a relatively good amount of sun hours in the required heating/cooling season.
There are two key concepts to this proposed heat pump setup. One is the use of a variable speed DC→AC solar inverter to run the compressor-motor. The second concept is the use of tanks filled with water as a thermal storage device, which eliminates the need for a large bank of batteries.
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Solar panels
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DC→AC converter
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Metering device (expansion valve)
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Compressor and motor
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Hot tank
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Cold tank
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3-way valve
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Circulation pump
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Outside thermal exchanger
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Underground thermal dump piping
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Indoor thermal exchanger
Figure 1 shows a basic overview of the main components. In this overview both the evaporation copper line (cooling) and the condenser copper line (heating) are inside the water tanks. This has both advantages and disadvantages. For the Australian system, an external heat exchanger was used for the hot side, this is because cooling in the summer was more of a priority.
The water in the tanks is either heated or cooled during the sunlight hours using the heat-pump. Because water has a high specific heat capacity, it makes it attractive for thermal storage of both hot and “cold”. Water also has the advantage of being environmentally friendly, inexpensive, and user-friendly when it comes to transporting thermal energy around a building. Because water is used as the transport medium, the required refrigerant-gas volumes are greatly reduced. This reduction can be over 75% when compared to Variable Refrigerant Flow (VRF) systems.
DC→AC frequency solar inverters are commonly used for electric motors that drive water pumps in isolated settings where there is a desire to avoid the need for batteries. The inverter is installed directly between the solar panels and the electric motor. It then adjusts the RPM of the motor based on the available solar energy. This allows the motor to run at full speed when there is plenty of sun but at a lower speed early/late in the day or when there is cloud cover. This results in a much higher total runtime throughout the day.
In a heat pump, the motor that is turning the compressor (compressor-motor) accounts for roughly 95% of the electrical energy consumed by a heat-pump unit. It makes sense then to run this motor only when there is an abundance of solar energy. For years the compressor-motor market has been dominated by the electrical asynchronous motor. The asynchronous motor is one of the world’s most common electrical motors. In addition to its extensive use in HVAC systems, it is also found in a large range of other equipment. In recent years, more efficient motors have entered the market, but the asynchronous motor is still a very reliable, efficient, and durable unit. This is why this type of motor was selected for both the project in Sierra Leone and Australia, more motor related information can be found in section 6.
3. System in Sierra Leone
I installed this system without access to a vacuum pump or gauge set and the heat exchanger was made using a Land Cruiser radiator (condenser) in a water bath. I also had to make a very basic metering device (see Figure 3), which is not recommended but it worked okay. I was transferred to another country before I had the chance to fully finish this project. But I did get it running, which allowed me to learn a lot and prove the concept worked. A description of the system is below.
Having modular-based and low-tech systems in these settings is often a big advantage. As automotive air conditioning compressors are belt driven and low tech, this is the compressor that was selected. These compressors are surprisingly powerful based on the air volume of a car’s interior - reasons for this can be googled.
The main focus for this system was cooling, but hot water was produced as a waste product. This hot water could be used for showers, washing machine, etc.
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4000l water tank covered in old medical box insulation
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Motor and compressor
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Water box with Land Cruiser radiator (condenser) inside
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Four old outdoor units with compressors removed
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DC→AC inverter, main cut-off switch and lightning arrestor
The light blue box (#3) was filled with water and functioned as a heat exchanger. Heat was transferred from the refrigerant-gas to the water. The hot water was then pumped through the four outdoor units. These four units were modified to allow for a higher flow of water by reconnecting some of the radiator sections into parallel and removing the metering device. A coil of copper pipe (evaporator) was installed into the 4000L water tank and used to cool the water. This cold water was then pumped through insulated hoses/pipes to old indoor head-units to cool the building.
Items used in this project.
4. System in Australia
This second version of this system was built in Australia and was more advanced because I had access to more parts and equipment. I also had the experience and lessons learnt from the system in Sierra Leone. However, I still wanted to keep it as low tech as possible and try to use items that are commonly available around the world. Again, I used an automotive A/C compressor, which was removed from a Toyota Prado that had driven 400.000kms (250.000 miles).
The goal of this system was both cooling and heating, so two 1000L tanks were installed. However, like the first system, a coil of copper pipe (evaporator) was used to cool down the water inside one tank, whereas the second tank had an external heat exchanger feeding hot water into it.
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The metal frame for the motor and compressor was placed on a rubber mat and later buried with sand and gravel
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The heat exchanger is sitting on and leaning against some insulation, it was also covered with insulation later.
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Cold water tank (IBC) covered with insulation
A second-hand air handling unit (Figure 9) was selected instead of the head-unit that was used in Sierra Leone. This was installed under the house and then ducting was run up through vents in the floor. By closing and opening 4 valves, it was possible to change between the hot and cold-water tanks that this air-handler was fed from, and therefore switch from blowing hot air to blowing cold air. Total room size that was warmed up was over 500m3 (17,600ft3).
Having a closed loop water system always runs the risk of having algae buildup. Figure 10 is a picture taken of the cold-water tank after 6 months of use, the hot tank was just as clear. No chemicals have been added during this period. The lids on the IBC tanks were left a bit lose to allow for pressure changes. It is possible to see some white foam where the copper pipe passes through the tank bulkhead. This is done so that water cannot freeze around the copper pipe at this location and then crack the bulkhead.
5. Things to consider
Here are some thoughts based on my experiences from the first two setups and also other improvements that I think can be made. Because both systems detailed in this document use an automotive compressor, this will be the basis for further discussions. However, using a hermetic compressor would not change much.
Selecting a site for the compressor and tanks
To keep refrigerant gas volumes low, it is recommended to keep the compressor as close as possible to the water tank(s). However, moving the tank(s) and compressor away from the building should not create problems as it is quite easy to pump water from one location to another, but more pipe insulation will be needed. It is good to put some planning and thought into where the solar panels and inverter will go and the weight of the water tanks.
System sizing
The compressor should be selected based on the needs of the building. Then a 3-phase motor can be selected based on the size of the compressor/metering device. Welding up a solid steel base with belt adjustment for these two items is highly recommended as it will absorb the forces of the belt system and help to reduce vibrations. As mentioned before, making an automatic belt tensioner with a gas strut instead of a spring eliminated the vibrations on the Australian system.
Solar and inverter
Based on the size of the motor, an inverter needs to be selected, and then based on the inverter, solar panels can be selected. Most inverter manufactures recommend the total kWpv be 1.5 times the inverter size - see section 6, “Motor Speed and kW” sizing for more details regarding this topic.
Days with poor sun
If using the system for cooling, then generally days without much sun will be cooler which will help counter the lack of production. However, when running the system for heating, the days without sun will compound the problem. This could be offset by having a larger storage capacity for the hot water tank, but this would need to be factored in when choosing the sizes of the compressor, inverter, and solar array. In theory, it would be possible to use a very large tank for even longer periods without sun.
Nighttime cooling of the hot tank (summertime)
If your daytime air temp is for example 38°C (100F) and you are using the system for air-conditioning and want to increase its efficiency, then you can use the nighttime air to pre-cool the hot tank for the following day. This would be beneficial in the following way; if your hot side tank is around 50°C (122F) at the end of the day and your nighttime temp is around 22°C (71F), you can run your outside fan-radiator and circulation pump to bring this water down to maybe 25°C (77F) during the night. This is a big benefit for a small contribution of electrical energy in the form of pump and fan. If dimensioned correctly, it would mean that the next day you are cooling your hot-side (condenser) with water that is colder than your outside air-temp. There are a few other things that need to be taking into consideration with this so some thought needs to be put into and I will post a bit more about it at a later date.
Noise level
I was afraid that the unit would be much noisier than other outdoor heat-pump units. It was noisier, but a lot of this was high frequency which is easy to block with some sound deadening insulation. When doing this make sure that the motor still gets enough air flow for cooling.
Water to ice
It is possible to significantly increase the thermal capacity of the cold tank when running the system for cooling. This is done with a phase change from liquid to solid (water to ice). Achieving this change allows for a reduction in the needed volume of water for cooling as well as stabilising the water’s temperature while there remains ice present in the water. Clearly not all the water in a tank can be converted to ice because liquid is still needed as a transport medium.
Evaporator configurations
In both systems, I used a single coiled copper pipe to distribute the “cold” in the tank. One problem is, as soon as a layer of ice is created on the surface of the copper it starts to insulate the copper - the more ice, the more insulation. One way to combat this would be to divide the single copper pipe into 3-4 smaller but longer copper pipes. However, another way to combat this, which would be especially helpful in the summertime, would be to create an ice making plate at the bottom of the tank. This would be a more complex system and would require a hot-bypass valve, but it has quite a few advantages. More information will be published on this once I have added it to the system in Australia.
Below freezing environments
If you are living in an area with extended periods of sub-freezing temperatures, then having the water running through the outdoor radiator unit will create problems. This can be addressed with an extra heat exchanger and running glycol in the outdoor unit or running glycol in the whole system. Running glycol in the whole system will be costly and worse for the environment if there is a leak. Another way to address this would be to increase the capacity of the underground piping or move the radiator unit into a small building and duct in and out the air flow, but this only works if the day temp is still over freezing.
Heat/cold dump
If you are using the earth to dump your unwanted heat or cold, it is good to select an area that is not sandy and has a high humidity level. This could be near the overflow of a tank or the exit of a grey water system. It is also possible to use a stream or other natural body of water, but care should be taken not to heat/cool so much that it affects the natural environment.
Types of insulation
If the system will be used for air-conditioning it is important to use an insulation material that does not absorb water. The reason for this is to stop condensation soaking the insulation material and drastically reducing its insulation value. When insulating hot pipes or tanks there is no need to worry about this.
Water piping
When using water as a thermal transport medium, it is important to select hoses and pipes that can handle the temperature. The maximum temperature will depend on the gas type used and the minimum temp will be freezing because you can only go below that by running glycol. Also, make sure that you have enough separation between hot and cold pipes, particularly at the location of the 3-way valves. If you have very short pipes on these, then you might be heating your cold water and cooling your hot water because the valve is very close to the tank for example. Also, important to try and keep the pumps as low as possible in the system, this will ensure that they have enough head-pressure to reduce the chance of cavitation.
Storage capacity
Depending on the regional climate it might be that the thermal energy is being used while it is being produced. This would be typical in the summertime when the hottest part of the day is also when the sun is shining. However, if it was in the wintertime, this thermal energy is stored in the form of hot water until later in the day when it is cooler or when people are home from work. This is one reason it can be beneficial to have more storage capacity for hot water than for cold water. The second reason is because of the possibility for phase change in the cold tank (water to ice), as discussed earlier. Some companies are using sand as a thermal heat battery; however, this will not work as well as water does while so long as the temperature is under the boiling point of water.
Controlling and monitoring the system
If you would like to set up the system in its most basic form, then there is really no need to have a control system. There wasn’t one installed on the system in Sierra Leone. But if you would like something more advanced, like the system in Australia, then you need to have sensors, relays, timers, etc, to follow what is going on and to start/stop various pumps etc. For this, I used an Arduino: they are inexpensive, reliable, and quite powerful. They need to be connected and programmed but it is not high-level stuff. If you have not done this sort of thing before, then you can often find someone online that can help you with it.
I bought and planned to install many temperature sensors for the system in Australia but did not get time to install them before I left. They will be installed later this year which will make the system more efficient, as well as provide data on how many kWs are really being put into or taken out of the water.
6. Extra info
Alternative heating/cooling
In some cases, I would not recommend using this system and rather go for something that is cheaper and simpler. If you need cooling in a region with low air humidity and can spare a few litres of water a day, then evaporative cooling is worth looking at. If you need heating and still have a fair amount of sun, then it could be worth looking at Solar Thermal Panels and then storing the heat in insulated water tanks.
DC electric motors
One option that I have not looked into would be using DC motors to run the compressor, eliminating the need for the DC→AC inverter. However, other factors need to be considered if doing this. Also, large DC motors are not as common as large AC motors, so I have not looked further into this.
Motor Speed and kW
Some unconventional thinking maybe but detailed below are the reasons for some of the choices made regarding motor speeds and sizing. A lot of this is linked to how many kWs an automotive compressor actually consumes. Finding reliable consumption data for this proved to be more of a challenge than expected. So, some calculations are based on “gut feeling”, and some linked to the basic testing done while the compressor was still in the vehicle.
Typically, these compressors will be powered by a crank RPM between 700rpm and 3000rpm for this type of automotive diesel engine, however it can be higher for short periods. Installing the crank pulley from the diesel motor onto the electric motor meant the desired standard RPM for the electric motor should lie between 700-3000rpm. In the end, I selected a 1440rpm motor for both systems, but the plan was to run it above its rated rpm.
I decided to go for a motor with a power rating of 4.5kW, this would have then meant a (minimum) 4.5kW inverter. Based on the inverter manufactures x1.5 rule of thumb a 6.75kW solar array would be required. However, this means the peak production of 6.75kWPV would not all be used, as there are no batteries in the system and the motor is only 4.5kW this extra potential energy is never used. I was quite sure the capacity of the compressor was greater than 4.5kW. One way to get around this is to run the motor faster; a motors kW rating is also linked to its rpm rating which again is linked to the 50 or 60Hz power supply that it was designed to run on. Therefore, if you run a motor faster, it will also deliver more mechanical energy. I actually put the upper Hz limit for the inverter at 90Hz and therefor let the maximum output of the solar system determine the resulting RPM of the motor (ca 2600). This should not be done if you are wanting to stay on the safe side of things.
Grid tied stability
As grid-tied solar installations are becoming more common, they are also changing the way power companies need to manage demand and supply. For example, if there are 100 houses in a suburban area and 70% have battery-less grid-tied solar, then on a sunny day when everyone is at work there will be a lot of production from these houses, but often not a lot of consumption. The opposite is true when people get home from work. Production will be lower, but consumption will be higher. If the power being consumed in the houses is for heating/cooling, then the system proposed in this document could help with this imbalance.
Compressor type
Belt driven (automotive) A/C compressors have been used for both projects. They are very cheap or free, they are simple to work with and use, and often have O-ring ports rather than welded joints making some operations easier. But there is no reason why a hermetic 3-phase scroll (or other) compressor cannot be used. Another option would be to look at an electric car A/C compressor. Most of these have bush-less DC motors, so a different type of solar pump inverter would be needed. Having the compressor and motor as two separate units connected by a drive belt has the advantage of not transferring heat from the motor windings to the compressor. This is a good thing when using the system for cooling but is a disadvantage when using the system for heating.
Bigger systems
If it is needed to cool/heat a lager house or building, then using a bigger compressor is a possibility. A mini-bus, truck, bus, or even a refrigerated shipping container would be possible. Another option is to run two compressors off one bigger motor. Then, the inverter and solar array would need to be sized accordingly. Even though this would complicate heat-exchangers and other piping, it would give the advantage of cutting out one of the compressors when there is heavy cloud cover or early/late in the day and this would help hold an acceptable level of RPM for the compressor that is still running.
Power supply from other renewable energy:
It is also possible to use the system with other renewable energy sources. But this would only be of benefit if this source was intermittent, or it was small and the tanks were used as a buffer system.
7. Conclusion
The system in Australia will be updated with several temperature sensors increasing the efficiency, as well as providing data on how many kWhs are really being put into or taken out of the water. I will also try and change the configuration of the evaporator to a plate in the bottom before the end of the year.
Many specific details were left out of this document, with the goal of providing a short overview. If you are interested to learn more about the projects mentioned here or see how the updates go, find the newest information on the website, Facebook and Instagram.
https://www.facebook.com/profile.php?id=61552029121584
https://www.instagram.com/hatfieldinnovations
Acknowledgements:
I would like to thank the following people for help in various ways on this project.
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Pete Brindle
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Gunnar Harry
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John Lane Refrigeration
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Douglas Industries
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Chill-Rite Refrigeration and Air Conditioning
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Lysaghts Refrigeration and Air Conditioning
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NordDisk AS
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Also a big thanks to the people that helped with drawings, proof reading and translated the document into an additional 4 languages.