Tuesday, October 8, 2013

Connecting Technologies - Part 2: Aero-Solar System


In the previous post we talked about how connecting technologies can create a new quality on the example of a turbofan engine which made possible modern commercial aviation.

I dare to claim that the same is just about going to happen with integrated building energy technology.
Let's look for example at today's solar thermal systems. Electromagnetic energy of solar radiation is captured by solar collectors, transformed into thermal energy of the heated core, and passed to the domestic hot water system via heat exchanger. The pump circulates the water in the system passing this thermal energy to the storage.


It would be nice if the sun would never set and we could collect its "free" energy 24 hours a day. As we all know, this is not the case. The stubborn sun doesn't want to shine all day. Worse even, it circles across the sky, it rises and sets at different time in winter and summer, not even talking about clouds and other nature phenomena, making it not possible to consistently collect energy during the day. At the best, we can count on taking the most of it but for one brief moment when the sun is directly above the collector (and even then, only if we tilt it at the right angle - which is a subject of a separate discussion).  Therefore the energy we can in theory collect during the day would have a profile something like the one shown here in dotted line. In practice, due to the delayed heat loss it might be something like a solid orange line.


However, what if the actual demand for hot water is not when we have the most energy available?
And indeed the typical household hot water demand is almost inverted to the energy profile. In the morning we wake up, take a shower, make breakfast and go to work. There is a morning spike in hot water consumption.  We usually not home during the day - at work, at school and other business - thus the consumption through mid-day is near-zero. Then we come home, we have a dinner, we turn on the dishwasher machine, we do laundry, take a bath - another, larger spike in hot water consumption.

 
Comparing both graphs, one can easily see that peaks of one correspond to the troughs of another! Classical economics question: how to match supply and demand?
One solution is to accumulate the excess energy when we have the peak of its availability and use it when we don't have enough, say in the evening. Sure, a hot water storage tank may do the work. This is what is used in almost all domestic hot water systems, regardless of the source of energy, either conventional gas or electric heater or alternative, like solar or geothermal. Speaking of the latter, this can be a consistent source of energy (shown in the solid black line on the previous graph), since the temperature under ground does not depend on the sun position or the weather - a couple of meters down from the surface it is practically constant throughout the year. The problem is in order to obtain enough energy, it requires drilling, which is often not possible in the urban conditions, sometimes not allowed for environmental reasons and is always expensive.

Back to solar. Many locations in Canada enjoy abundance of solar radiation, especially in summer.


Solar thermal system, on average, is the most cost-effective renewable system. It provides much higher efficiency than solar photovoltaic systems (the latter involves conversion of solar radiation into electrical energy, and any conversion means a loss of efficiency). It would not come as a surprise however, that  due to shorter days and lower sun, the energy coming from sun in winter is much less than in summer. If we want to match the summer peak demand, an example of this kind of energy balance over 12 months might look something like the graph below.

    
But we also want to take shower in winter - may be even more than in summer! How we should always match the demand? Of course, we can add more solar collectors, increasing the total energy collected to the point when we match the peak demand in the worst winter time. Then the energy balance may look something like on the next graph.


One can easily see that even if we manage to match the peak demand for any given moment, for the most part we collect much more energy than we are able to use, so we will need to dissipate (dump) it. Overcapacity is not a very sustainable way of building sustainable system, is it? Not even talking about additional cost for extra-collectors and, in most cases, simply impossibility to allocate a large enough area which would be required to accommodate that many collectors.        

We want to size the solar thermal system in such a way so to collect energy when we have an excess of it comparing to the demand to be stored and used when there will be lack (or absence) of the usable solar radiation. But how to choose the right storage capacity?  
A high-capacity thermal storage capable to accumulate large amount of thermal energy and be efficient enough not to loose it quickly, may require a large super insulated tank, ideally underground, which may be impractical or otherwise expensive.

 
The answer comes again in the form of integrated system. Air-to-water heat pumps (a.k.a. active thermal exchange) are becoming more and more efficient reaching COP 5-6 and more, meaning they can generate 5-6 times more energy than they consume. Combined with the solar thermal collector and high-capacity and low-loss thermal storage, controlled by the program using adaptive algorithm, this integrated system can satisfy demand at any time cost effectively, with high reliability, requiring little physical space, and using no fossil fuel.   




Ascent Systems Technologies, with support from National Research Council of Canada, has developed a program called ASPA (Aero-Solar Predictive Algorithm). It does exactly that - automatically chooses the most optimal parameters of the integrated hydronic system given the location and the actual demand.

2 comments:

  1. I greatly like the diagram shown in here - very well-presented.

    Point Of Purchase

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  2. Thank you for providing the information. I would like to see some more blogs on this topic.. solar pv

    ReplyDelete