Pumped Heat from the Tunnel

The idea to use a reservoir of water as a heat pump’s heat source is not new. But now and then somebody dares to do it again in a more spectacular way. Provided governmental agencies give you permit, lakes or underground aquifers could be used.

Today a (German) press release about a European research project called Sinfonia caught my attention. The cities of Innsbruck (AT) and Bolzano (IT) plan to reduce energy demands by 40-50% and increase the percentage of renewable energies used for heating and electrical power by 30%. The results should serve as best practices applicable to other cities.

Diverse activities are planned, such as improving insulation, installing solar thermal collectors or photovoltaic panels, and developing ways to renovate even buildings that are subject to monument protection. The latter is quite a challenge in European cities as laws do typically not allow for installing anything that impacts the view of historical rooftops or the structure of facades.

In a smart grid infrastructure, energy demands and supply should be managed, for example by cooling down refrigerators to -30°C if too much electrical energy is available – effectively storing energy in the ice.

My favorite: Heat pumps should utilize water from a very special source – drain water from the tunnel underneath Brenner pass (called tunnel water or mountain water in German).

This source would provide water flowing at 200-300 liters per second at a  temperature of 22°C, resulting in about 10 Megawatt of heating power.

I want to cross-check these numbers:

Assuming low-temperature floor heating loops, heat pumps would need to operate between 22°C ‘input’ temperature and about 40°C ‘output’ temperature.

The maximum theoretical efficiency is limited by principles of thermodynamics: This is Carnot’s Coefficient of Performance, which is:

Thot / (Thot – Tcold)

There are absolute temperatures in Kelvin, so 273 K needs to be added to the temperatures in °C.

Thus the COP is about:

COP = (40 + 273) / (40 + 273 – 22 – 273) = 313 / 18 ~ 17

Carnot’s perfect circular process does not include any phase change – as the evaporation and condensation in a real heat pump – and there are different sources of energy loss.

Thus a real-live heat pump shows a much lower COP. But there is a simple rule of thumb based on experience that is surprisingly accurate: Divide Carnot’s COP by 2 to calculate the realistic COP.

So these heat pumps would operate at a COP of 8,5 which is still very high. The temperature of the tunnel water is expected to be rather constant – as ground water – so COPs will also be high in winter.

Standard ‘geothermal’ brine-water heat pumps show COPs of about 4 – 4,5 when operating between the standard temperatures (values used in standardized tests) of 0°C brine temperature and 35° heating water temperature (B0/W35). As we discussed the meaning of ‘brine’ in the comments recently: In relation to heat pumps this term always refers to a solution of glycol-based frost protection in water.

Water-water heat pumps utilizing ground water with a temperature of about 10°C show a COP of 5,5 to 6 (W10/W35).

I have picked 40° rather than 35° in my estimate, accounting for losses in a district heating system attached to the gigantic heat pumps. Had I calculated with 35° my numbers for the tunnel water heat pumps would even be higher. So the whole exercise is more of an order-of-magnitude check.

The mass flow of about 300 kg (= 300 liters) per second can be converted into power retrieved by the heat pump: by multiplying it with the specific heat of water (4,19 kJ/kg) and with the temperature drop of the water caused by passing the heat pump’s evaporator unit.

The temperature difference in the brine circuit connecting the heat source to the heat pump is about 5 K for standard ‘small’ heat pumps. Water-water heat pumps might also use and additional brine circuit and thus an additional heat exchanger to transfer heat from the source water to the heat pump. I use those 5 K nonetheless as it is not a constant anyway – mentally insert error bars of several 10% here.

The drain water ‘carries’ approximately:

300 kg/s * 4,19 kJ/kgK * 5K ~ 6285 kJ/s = 6285 kW ~ 6,3 MW

The COP represents the factor the electrical energy feed into the heat pump is multiplied with to yield the heating power.

[Heating power] = COP * [Electrical power]

Given a COP of 8 an electrical power of 1 MW would result in 8 MW of heating power, delivered to floor heaters for example. However, the remaining 7 MW then need to be retrieved from the heat source, and the heat source really needs to be able to deliver them:

[Heating power] = [Power retrieved from source] + [Electrical power]


[Power to be retrieved from source] = [Heating power] * (COP – 1)/COP

Aiming at 10 MW heating power output using a heat pump with a COP of 8, the drain water would need to deliver:

[Required power from drain water] = 10 MW * 7/8 = 8,8 MW

This is higher than the calculated power of 6,3 MW but the temperature drop I used was just an estimate and 7 K would also as well be OK – let alone all the other assumptions for operating temperatures and COP. So the numbers from the press release are self-consistent. 

11 Comments Add yours

  1. So, having read the link to the Brenner Pass, folks are drilling through the Alps to produce a transit tunnel. The project will understandably liberate a tremendous underground water source, and this will be used as an input source for heat pumps. I had to questions. First, how will this input source be distributed? By something like a municipal drinking water system? And, also, the input temperature seems awfully high (22C/72F) … I usually think of the temperature of groundwater as bring closer to 5C/40F or 10C/50F degrees. Anyway …. it sounds like one of those win/win situations. With that flow rate, has anyone considered the fact that the water can be used to first drive turbines and then distributed to supply the input source for the heat pumps … this way they could double the energy output … one supply relies on the flow rate of the water, while the other relies on the temperature differential. Now I’m walking like an engineer! D

    1. elkement says:

      Excellent questions, Dave!!

      1) Distribution: The press release does not make that entirely clear, probably this is to be developed as part of the project. But typically, these are the options
      — Have some ‘big’ heat pumps located in a central place and distribute the hot water via local district heation (quite popular in Austria for local biomass-powered plants) or
      — distribute the tunnel water (or brine of about the same temperature) to homes in a “low temperature” district heating system. The latter seems more cheaper at first glance as thermal losses will be lower – but on the other hand operations of one central heater is often cheaper if you factor in the investment costs of all the separate small heat pumps. With district heat there is usually an “economical optimum” that determines the design.

      2) Tunnel water temperature. It depends of the depth beneath the surface the water originates from. As a rule of thumb, the temperature of the first ~ 10m is determined by the radiation of the sun, then it stays constant for 100m at 10°C. But below that, the effects of (‘real’) geothermal energy make the temperature rise, at about 3K/100m. The exact value varies locally, and it happens to be extraordinarily high in Northern Italy (Tuscany).

      3) In hydro power plants energy is gained from water flowing over a steep gradient, thus from potential energy – but not from the component of the velocity parallel to the surface using sort of a ‘wheel’ just immersed in the water … but I am aware of some innovative ideas for using floating mini-power plants (shaped like little torpedos) that extract some kinetic energy from the Danube for example.
      I had now started to do some ad hoc calculations but I better cut and save this for a future blog post. Bottom line is that the kinetic energy calculated from the flow velocity and mass flow is rather low even though 300l/s might seem to be much at first glance. This also holds for large rivers when comparing the modest kinetic energy to the gains from potential energy used in hydro power plants, even if the latter just make the water ‘fall’ by 10 meters.
      In order to estimate the energy gain for a tunnel water hydro plant I’d need to make an assumption about a slope (classical power plant) or I need to assume that all kinetic energy is harvested, perhaps by having the water trickle away in some soakaway after the plant (velocity nearly zero) – maybe not that unreasonable for such a ‘ground water well’. But as I said, from some quick cross-check I conclude energy gains would be very, very modest in comparison to heat energy. Thus I believe building such plants would not be economically feasible – especially with so many real hydro power plants ‘nearby’.

  2. Over my way the focus is still very much on hydroelectric energy. The land mass that is Newfoundland Labrador is fairly large–5 times that of Austria, yet our population is disproportionately tiny–0.5 million as opposed to your 8.5 million. There are quite a few sources that can be used for hydro and they are largely untapped.
    Unfortunately in the last half-century we grew quite complacent and lazy here and stopped that sort of growth, focusing instead on the development of our offshore oil and gas. We even took to using an oil fired generator to power the population rich Northeast Avalon region. Pure bad planning.
    Topping that off is the SOUR deal we brokered with Quebec for the distribution of electrical energy from our massive Churchill Falls station via their transmission lines. The station produces around 5400 MW (yes!!), the vast amount of which is sold to Quebec in a binding deal for $0.002/kwH. That’s not a typo–two-tenths of a cent!! Quebec, as you imagine resells it at massive profit leaving NL with dirt.
    At the moment one expansion project is underway that will add another 1000 Mw or so and several more are planned. It’s hard, though, to secure financing and there’s always the issue of getting the power out through Quebec, which demands extortionally high tributes for that.
    Perhaps we here in this place need to reset our thinking and work, instead, on recovering the massive energy we already have from our bountiful groundwater…
    Food for thought, as always, Elke.
    BTW–I’m only about 1/4 of the way through the book yet; had a busy tiring week. Hope to make some headway this weekend.

    1. elkement says:

      Thanks – it is very interesting to compare the ‘cultural context’ of energy generation and transmission! Austria’s main source of electrical energy has always been hydro power, too, and we took pride specifically in pumped storage hydro power (used to balance and stabilize the system). Austria had been a net exporter of power until the installation of new power plants came to a halt when the utilities stopped investing after ‘unbundling’ (of power generation and transmission) had been enforced at the change of the millenium.

      I can unfortunately relate to your province’s weird deal: Here, some local governments made deals such as ‘sales and lease back’ their power plants to US entities (!). If I understood that correctly it was sort of a win-win situation as it meant cash flow for governments and they helped exploiting a loop hole in US tax legislation.

      I am now over-generalizing but it seems to me that industry concerned with essential infrastructure that is or has been mainly owned by governments is often prone to, let’s say, not the most ethical or reasonable business practices. Not sure if this is due to the former monopoly.

    2. elkement says:

      … as for the book: I have devoured it immediately but I need to let it sink in and sort out my notes. I considered to review it in “this week’s post” but then I had been distracted by the tunnel-powered heat pump :-)

      1. I’m just past the mid-way point now. It’s been VERY busy between work and home. Besides, a book like that does need some to digest.

  3. M. Hatzel says:

    I should mention in addition to the comment that I like the appearance of your newly redesigned solar collector!

  4. M. Hatzel says:

    “The latter is quite a challenge in European cities as laws do typically not allow for installing anything that impacts the view of historical rooftops or the structure of facades.” — I wonder what kind of innovations this will generate? In an ideal world, such an initiative might spark a change in either products or attitude so that solar panels and wind turbines are no longer regarded as unattractive.

    1. elkement says:

      You are absolutely right! I had for example seen:

      –PV solar panels that weren’t blue but reddish or greenish and including translucent parts. This makes the efficiency worse but those would make for much more beautiful parts of facades. Recently I have come across completely translucent ones that were used as roofing for the patio of a well-known castle in Vienna.

      –Hollow roof tiles made from metal, manufactured by a spin-off of a company selling metal to industry. These tiles were meant to serve as heat source (for heat pumps) in protected house where it was not possible to install any other heat source – so they would have replaced both tank and collector or ground loops.

      1. M. Hatzel says:

        I wonder if there is a niche for a small start up company? Or a side-line for one that already exists? ;)

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