I am often asked by people who are designing or renovating their homes whether a ground heat exchanger (GHE) is cost-effective. It is a good question, but there is no one-size-fits-all answer without a thorough examination of the topic. Let's take a look at a specific case study to determine if a GHE is cost-effective in this scenario. For other buildings, the analysis may be different and should be approached on a case-by-case basis. This analysis provides a general overview of the issue and will help many people make a reasonable decision.

However, we must make certain assumptions before conducting this analysis.

ASSUMPTIONS:

The building under consideration will be a single-family home with an area of 150m2 and a volume of 300m3. The efficiency of household ventilation calculated according to the PN-B-3430 standard is 150m3/h, which corresponds to 0.5 air changes per hour.

GHE - there are many types of GHE, such as gravel, plate, glycol, or pipe heat exchangers. Since I only have access to an excellent producer program for pipe heat exchangers, which can actually calculate the GHE, I will use the AWADUKT pipe GHE from the Rehau company. To avoid elaborating too much on the heat exchanger itself, I refer to the manufacturer's website, especially since my goal is not to promote any particular GHE GPWC ground air heat exchanger. REHAU Poland's offer. Furthermore, I assume that the GHE will be arranged in a ring around the building at a depth of 1.5m, and the total length of the DN200 pipe will be L = 50m and 12 45-degree elbows. The type of soil is clayey soil.

GHE cost:

a. DN200 pipe with antibacterial coating - 8 sections of 6m (6x1145zł) + 1 section of 3m (1x650) = 9810zł

b. 12 45-degree elbows (12x188zł) = 2256zł

c. 5 sleeves (5x172zł) = 860zł

d. DN200 ground air intake - 3127zł

e. Price of 1m of mechanical excavation ~150zł/m (50x150zł) = 7500zł

Total material cost 16053zł

Total excavation cost 7500zł

Combined cost 23553zł

The building is serviced by a good rotary heat recovery ventilator with a constant seasonal heat recovery efficiency of 85% and an electric heater. The rotary heat exchanger does not frost, which is the main disadvantage of a cross-flow or counter-current heat exchanger.

Electricity price 0.77zł/kWh

GHE operating time in winter from 01.10 to 31.03

Katowice City - Climate zone III (te=-20st.C)

Ventilation operates continuously, which is, of course, a less realistic operating scheme for ventilation, but for the purpose of this analysis, it is assumed.

With the above assumptions, we can proceed to the main calculations:

Enter the above calculation assumptions into the AWADUKT Thermo program.

2. After clik calculation we have results as below

So the average seasonal efficiency of GHE is 21.5%, and the pressure drop on GHE is 13.8Pa, which will increase the energy consumption of the heat recovery ventilator, of course.

The formula for the total efficiency of the air heat recovery system, taking into account GWC, is as follows:

So if the heat exchanger has a heat recovery efficiency of 85% and GWC of 21.5%, the total efficiency of the heat recovery system will be:

**Całkowita sprawność 88,4%**

Total efficiency 88.4%

Now let's see how this translates into operating costs of the heat recovery ventilator (calculations performed using the KZE module of the IX-CHART program):

Calculations for a heat recovery ventilator without GHE (total heat recovery efficiency of 85%)

2. Calculations for a heat recovery ventilator with a Ground Water Cooler (total heat recovery efficiency of 88.4%). Here, for the supply fan, I am also adding 14Pa of hydraulic resistance from the Ground Water Cooler.

To summarize, the cost of operating the heat recovery ventilator without a GHE would be around 742 PLN per year, whereas with the additional GHE it would be 643 PLN. The annual savings would be 742-643=99 PLN. To relate this to the simple payback period (SPBT), which determines how long it will take for the invested capital to be recouped in operational savings, we have: SPBT = installation costs/annual savings = 23553/99=238 years.

The conclusions are rather obvious and do not require additional comments. To be less strict in my evaluation, I assumed that the GHE would be made of cheap materials purchased on Allegro, with the cost of the corrugated pipe being 3500 PLN, the intake 500 PLN, and the excavation, laying, and backfilling 5000 PLN. This gives us a total of 9000 PLN. Even with these parameters, the SPBT would be 91 years.

In other cases, the results may, of course, differ from each other based on the input data. However, I do not think that this solution would bring tangible savings and economic sense, especially since for almost 24,000 PLN we can install a photovoltaic system with a capacity of about 5 kWp, which will generate 5000 kWh of energy per year, which will significantly exceed the savings obtained in this way.

Assuming that the price of 1 kWh of energy is 0.77 PLN (and it will probably be much more expensive), the annual savings would be 3850 PLN, so the SPBT in this case would be 6.1 years and without any subsidies. I assume that this post will not make me a favorite of GWC manufacturers, but my goal is to show a fair analysis.

What do you think about GHE and its profitability? - I would be happy to hear your opinion on this topic, so please do not hesitate to write in the comments.

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