Economics of the Solar Air Collector

In the previous post I gave an overview of our recently compiled data for the heat pump system.

The figure below, showing the seasonal performance factor and daily energy balances, gave rise to an interesting question:

In February the solar collector was off for research purposes, and the performance factor was just a bit lower than in January. Does the small increase in performance – and the related modest decrease in costs of electrical energy – justify the investment of installing a solar/air collector?

Monthly Performance Factor, Heat Pump System
Monthly heating energy provided by the heat pump – total of both space heating and hot water, related electrical input energy, and the ratio = monthly performance factor. The SPF is in kWh/kWh.
Daily energy balances, heat pump system, season 2014-2015
Daily energies: 1) Heating energy delivered by the heat pump. Heating energy = electrical energy + ambient energy from the tank. 2) Energy supplied by the collector to the water tank, turned off during the Ice Storage Challenge. Negative collector energies indicate cooling of the water tank by the collector during summer nights. 200 kWh peak in January: due to the warm winter storm ‘Felix’.

Depending on desired pay-back time, it might not – but this is the ‘wrong question’ to ask. Without the solar collector, the performance factor would not have been higher than 4 before it was turned off; so you must not compare just these two months without taking into account the history of energy storage in the whole season.

Bringing up the schematic again; the components active in space heating mode plus collector are highlighted:

Space heating with solar collector on, heat pump system punktwissen.
(1) Off-the-shelf heat pump. (2) Energy-efficient brine pump. (3) Underground water tank, can also be used as a cistern. (4) Ribbed pipe unglazed solar collector (5) 3-way valve: Diverting brine to flow through the collector, depending on ambient temperature. (6) Hot water is heated indirectly using a large heat exchanger in the tank. (7) Buffer tank with a heat exchanger for cooling. (8) Heating circuit pump and mixer, for controlling the supply temperature. (9) 3-way valve for switching to cooling mode. (10) 3-way valve for toggling between room heating and hot water heating.

The combination of solar collector and tank is ‘the heat source’, but the primary energy source is ambient air. The unglazed collector allows for extracting energy from it efficiently. Without the tank this system would resemble an air heat pump system – albeit with a quiet heat exchanger instead of a ventilator. You would need the emergency heating element much more often in a typical middle European winter, resulting in a lower seasonal performance factor. We built this system also because it is more economical than a noisy and higher-maintenance air heat pump system in the long run.

Our measurements over three years show that about 75%-80% of the energy extracted from the tank by the heat pump is delivered to it by the solar collector in the same period (see section ‘Ambient Energy’ in monthly and yearly overviews). The remaining energy is from surrounding ground or freezing water. The water tank is a buffer for periods of a few very cold days or weeks. So the solar/air collector is an essential component – not an option.

In Oct, Nov, and March typically all the energy needed for heating is harvested by the solar collector in the same month. In ‘Ice Months’  Dec, Jan, Feb freezing of water provides for the difference. The ice cube is melted again in the remaining months, by the surplus of solar / air energy – in summer delivered indirectly via ground.

The winter 2014/2015 had been unusually mild, so we had hardly created any ice before February. The collector had managed to replenish the energy quickly, even in December and January. The plot of daily energies over time show that the energy harvested by the collector in these months is only a bit lower than the heating energy consumed by the house! So the energy in the tank was filled to the brim before we turned the collector off on February 1. Had the winter been harsher we might have had 10 m3 of ice already on that day, and we might have needed 140kWh per day of heating energy, rather than 75kWh. We would have encountered  the phenomena noted during the Ice Storage Challenge earlier.

This post has been written by Elke Stangl, on her blog. Just adding this in case the post gets stolen in its entirety again, as it happened to other posts tagged with ‘Solar’ recently.

6 Comments Add yours

  1. Until recently out electricity costs were so cheap that collector / pump systems such as yours were rare in my province. With the new Hydro development currently underway running badly over budget it’s likely our costs will double over the next 5 years and i can easily see room for research, development and marketing of your ideas here. I’ve said it before and i will say it again: it would be great if your system were more widely know in my place as it makes a LOT of sense here.

    1. elkement says:

      Thanks, Maurice! This is interesting – and sort of the reverse issue compared to Germany. In Germany costs of kWh electrical energy are higher than in any other country – even 150% of the costs in Austria which is weird. In particular, electricity is more expensive compared to oil and gas prices which have their record low now. Nobody would heat directly with electrical power 1:1 anyway, but people would rather buy a new gas or oil boiler instead of a heat pump, even if the system has a high coefficient of performance.

      However, I try not to focus on current prices and predictions too much – who knows how markets for energy will react in a few years. It’s hard to assign numbers to Black Swan events. Our system has been and is economical, but one of our other main motivators for building it was also the rather grey swan of a national crisis after a shortage of natural gas which was not unlikely sometimes when Russia and Ukraine had their yearly squabble.

      1. Indeed–and in particular I do wish that most of the people who speak to the market regarding energy would share your candor regarding our inability to make accurate forecasts about costs, usage and availability. What’s needed now in my place is a sensible strategy to get people to think in terms of more (I really want to use the word efficient here but you know that’s not physically correct) cost effective means of ehating houses; ones that can do the multiplier effect your system uses. For example, if government could find a way of providing rebates or even zero-interest loans that people could use to help pay for the retrofit, and incentives–maybe property tax breaks–for those who choose to build these systems into new homes.

        1. elkement says:

          I have now started to write a lengthy comment and deleted most of it. Perhaps it should become a post in its own right, perhaps not because it is too negative.

          The gist of it was that I feel ambiguous about green governmental incentives. Here in Europe there are lot of initiatives. One questionable effect is that it fosters ‘business ideas’ that are built on seizing subsidies, organizations who are very skilled in perpetuating their ‘research projects’ forever, defeating the idea of using subsidies to start up an idea that might not get funded otherwise. Practices like that were one of the reasons we left academia, and all my credibility today comes from my own skin in the game, that is building that stuff from our own private money and having declined to participate in funded research projects and the like. I take a rather extreme stance on that and time will tell if it was really wise.

          It seems to me that all that tinkering (by government) with what finally should be functioning market does not really work, again proving Taleb’s hypotheses of too much interventions and micro-management having unintended side effects, like people and organizations gaming the system way more than the whole thing helps, say, poorer people to insulate their houses or reducing overall carbon dioxide emissions.

          I also wonder if subsidies for some selected things (likely to become target of some sort of gaming, don’t get me started about the proverbial insurance dealer becoming small wind turbine contractor over night…) would get to the core of psychological effects in consumers’ decisions: A heating system and insulation has to pay itself off and people scrutinize economics to death. If people would apply the same scrutiny to purchasing their cars or TV-sets, they could perhaps easily pay their heating system or insulation for their home from their savings from that. There is also well-known rebound effect: As soon as appliances get more energy-efficient, people have more or larger ones. If we would be just contented with what was the standard 20 years ago – in terms of electrical gadgets per person, or amount of food and drink you expect to be ready and cooled in the fridge every moment – lots of carbon dioxide and energy issues would be solved.

          1. Michelle H says:

            Hi Maurice and Elke; your conversation is (as always) very interesting. Maurice reminds me that there remain many places in Canada where natural gas is not a practical energy source for heat, and I do know many people who do not have access to it, sometimes opting for the awful choice of burning coal or oil (both are dirty/smelly, with highly volatile markets here that make for some price fluctuations). Even if the original investment of a system like the heat pump is high compared to (some) other options, I would think that because the input costs are rather consistent, it becomes very useful for planning daily expenses in the upcoming years, especially if you want to have the freedom to reduce your income (such as in retirement, or changing careers).

            We had more insulation installed on the house last month, and I want to write a post about it… just had a lot to do lately, and can’t seem to get over here to post anything.

            1. elkement says:

              Thanks, Michelle! I forgot to mention wood and wood pellets as an alternative – the latter are very popular here as a ‘green’ heating system. But retooling your cellar to a ‘pellets chamber’ is also quite an investment. But as I understood, there are not much trees in the prairie, so all things wood are not an option anyway?
              Oil is nearly considered on par with gas here, in terms of convenience – with the exception of the storage room for the tank. I think you don’t notice the smell if the boiler of your central heating system is tucked away in heating room. What drives green enthusiasts here mad about oil (getting back to my rant about subsidies’ now…) is that the oil industry gives out incentives to people exchanging their heaters – and they call it ‘subsidies’; so superficially those ads look like those of announcement of governmental initiatives. Marketing oil as ‘green’, is amazingly possible – arguing that a new oil boiler has less carbon dioxide emissions than an old one, which is of course true but beside the point :-)

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