Heating the house with a geothermal heat pump with heat intake from a septic tank or sewer
Heating by a heat pump today is one of the most profitable methods, comparable in cost to main gas or coal heating. But one of the inexpensive heat pumps (hereinafter referred to as HP) of the air-to-air or air-to-water system is that they cannot be installed everywhere (due to the climate). It turns off at minus 30 gr. and at this low temperature, its effectiveness drops.
For uninterrupted operation of a heat pump, a ground or otherwise geothermal one is needed. As a rule, laying 200-300 m of pipes for heating the coolant is associated with a large amount of earthwork. Or with a large number of wells drilled.
To reduce the cost of these works, you can use a trench-pit laying:
Dig a 3 m trench with an excavator and use a pit drill to deepen another 1.5 m. Install heat exchangers in them (HDPE pipe on racks, tied with a spiral).
But it is most difficult to extract heat from dry soil. It is necessary to consider how much the total length of pipes will be needed for your area of \ u200b \ u200bthe house and its heat loss.
Today this geothermal heating system is not cheap. But with a difference from the cost of connecting main gas or coal-fired boiler in 200-300 thousand. rub. For a house worth $ 5-6 million rub. it's not that big of a difference. We are not talking about payback. We are talking about the initial investment and the convenience of the heating system.
There is an interesting idea of how to additionally extract heat from the ground. Before installing the concrete rings of the septic tank, place one of the heat exchangers under it or even inside it. In this way, the heat that will go away with the sewer masses will be returned back. But it will not work to completely heat the house from a septic tank. It can be frozen and the TN will rise. The point is in additional heat extraction. Schematically it looks like this:
In practice, it might look like this:
When cooling water, we take 1.17 W / liter per 1 degree of cooling. When the family discharges water into the sewer system on average 1 m3 / day and when the water is cooled from +20 to +10 g, we get 10.17 kW of heat energy per day, we can return it back. Plus the heat from the soil around the septic tank. Not bad in theory. In practice, the water quickly gives off heat to the soil and not to the heat exchanger. And it will not be desirable to pump out the septic tank to the end.
I have come across ideas to extract heat from compost, manure fillings and biogas plants. In them, bacteria raise the temperature to 30 grams. even in winter. And if you lower it to 20 grams. (without freezing bacteria), then this is also additional energy. For example, on farms.
It is another matter if the central sewerage system runs next to the house. The temperature in it can reach 20-25 grams. This is free heat and with such a temperature gradient there will be a huge COP coefficient with a value up to 4 (1 kW of electricity was spent - 4 kW of heat received).
The central sewerage system can be compared to an always warm river. The removal of heat from this stream will not be reflected in any way. It is only necessary to agree on the connection to the sewer of the probe, the heat exchanger of the required length. Utilities may require a project for this tie-in to their system. And maybe a fee.
Also, this scheme is suitable for those who have a river or pond nearby. But in this case, the project must be coordinated with the environmental authorities (if everything is done officially). And as a coolant, it will be necessary to use not ethylene glycol, but propylene glycol.
The reader will say, okay, drilling wells and laying heat exchangers is basically work, but the cost of the heat pumps themselves, automation with a compressor goes off scale. Imported models are expensive, I agree. But there is also a Russian-made TN.
Photo from the construction exhibition TH manufactured by a company from Krasnoyarsk http://гто24.рф
The price is 2-3 times lower than imported analogs and the service is understandable: something happened to the TN, a new one was brought and installed, and the old one was taken for diagnostics. There is no need to wait for months for the delivery of spare parts for imported models in the event of a breakdown.