As an engineer, I try to think of things critically. By that I mean I don’t take a lot a face value. I try to take a step back, look at the situation or problem, and make my own determination based on the evidence. As a power engineer, the topic of renewables is on my mind a lot. It is in the news, on TV, in politics, and a popular point of conversation. Unfortunately, while I enjoy the topic as a whole, I can get frustrated by the lack of understanding (or pretending to understand) and constant use of “buzz words” on the issue. There are a lot of people (politicians are the worst) that make important decisions on our energy without knowing anything more then the information they read on a brochure from a lobbyist or off the internet. So to perhaps help give a slightly better understanding on the issues, I present the following.
The most common topics on renewables that I think are misunderstood are as follows:
- Renewable Percentage Goals
- Affect of high penetrations of renewables on power grid
Renewable Percentage Goals
To fully understand this, lets look at the energy distribution of the US in 2016
In 2016, renewables account for only 15% of all energy production. Note that around half of that production comes from hydro-electric power. Many states are trying to achieve goals such as 30% by 2030, or 50% by 2050. Lets think about this critically. Only 15% of the power right now is from renewables. Of that, perhaps 8% is from solar. Keep in mind we will not be adding much (or any) new hydro, so count that number as static. So the only way we get to the 30% (and beyond) goal is to increase wind and solar. Now since percentages mean nothing without scale, lets look at some numbers. We have around 4,000 terawatt-hours of total consumption in the US (that is 4 to the power of 15, so basically a lot). Solar and wind make up around 340 terra-watts of that. To get to 30% renewables we would need to increase power capacity in the wind and solar sector by 600 terawatt hours. To make that scale mean something, the average wind turbine has a maximum output of 2 Megawatts. Assuming a capacity factor of around 25%, one turbine could produce around 4.4 million megawatt hours or 0.0044 Terrawatt hours. So to make up the 600 terawatt hour deficit, we need 136,000 turbines, give or take.
Let that sink in, 136,000 turbines. The average wind farm has maybe 200-400 turbines, so at that rate we are looking at maybe 450 new wind farms. I hope that helps you grasp the magnitude of that 30% renewables number. Now think of 50%, how much capacity is that? Well, not quite double the previous numbers, but close, so maybe 750 wind farms. Seem like a lot? It is. Keep in mind we are just talking capacity, less we forget about all the infrastructure we need to build to even access these new wind farms. The point of this little math exercise is to help give weight to these simple percentages we toss around. Before we all jump on the band wagon that we need to get to 30%$ or 50%, we need to understand what an undertaking that is.
Affect of High Penetrations of Renewables on Power Grid
As we saw in the previous paragraph, to meet some of the lofty goals set certain state and federal entities will be daunting to say the least. What also needs to be considered is what having that much renewable energy does to our power grid. I will do what I can to keep the engineering jargon to a minimum here…
One of the biggest and probably least understood issues of having a 30% or 50% renewable generation penetration is inertia. To help you grasp that a bit, think of a bike wheel and a steam roller’s roller. Spin both of them at the same speed. Now disturb the motion of the wheel or roller. Maybe hit it, shake it, throw something against it, any number of disturbances. What would happen? Chances are the bike tire will slow down, maybe stop, it will at least change speed, how drastic would depend on the disturbance. How about the steam roller? That roller will be unaffected, or minimally affected for any disturbance, save a very large one. The bike wheel has very little inertia, the roller has a lot. The power grid has a similar operation, just slightly more complex. We have thousands of rotating machine (synchronous generators) that rotate to give us power at 60 Hz (60 rotations per second). If we have a disturbance on the power grid, there is thousands of machine rotating in unison that can absorb the disturbance (like the steam roller). Renewables however have no inertia. So when we remove machine and add renewables, we are losing system inertia. Now is that a big deal? Well kind of. Our grid suffers from disturbances everyday, multiple times a day. Some of them major, some of them minor. We are able to ride through most of them because the machine power right through. At a 30% and especially 50% penetration, we will not be able to ride through as well. What does that mean? Possible power outages, equipment stress, and a lot of work for operations personnel. Now its not to say its not possible to get to 30% or 50% and have a well running power system, but it will not be easy and it will not be overnight. Grid modification, new equipment, and new operations procedures will need to be developed to make it a possibility.
Inertia is just one of many engineering challenges we face to continue to support higher penetrations of renewables. Some other challenges I encourage you to consider are:
- Storage: Storage is still not cost effective and completely sustainable. Batteries carry significant costs, are very non-ecofriendly to build, and do not have a proven reliability track record. This is changing, but we have a long way to go here.
- Spinning Resources: There will always be times when wind does not blow as much as forecasted, or clouds reduce solar output. When that happens, many times we have to rely on spinning resources or reserves. These are machines that are “on-call” and ramp up to generate as needed. Gas has been the main driving factor for the increase in wind generation. Wind is variable and if you were to look at a daily graph of wind output, it can be all over the map. To smooth that out, gas turbines ramp up and down. One thing to always remember is power generation must equal power demanded at all times. Not at easy feat by any stretch of the imagination. So for every 1MW of wind we want to count on, we need to have a percentage of reserves to back it up.
- Infrastructure: If you haven’t noticed already, typically the large solar and wind farms are a little bit out of town…and by a little bit I mean very far out of town. Typically the areas where there is that much land available don’t have huge cities. Therefore there is a good change you don’t have many (or any) high-voltage lines you can connect into. How big of a deal is that? Here is a nice back-of-the -napkin, want to build a 230kV line? Knowing nothing else and assuming its overhead, you are looking at 230,000 x 1.5 = $345,000 per mile. In some cases that factor of 1.5 can be 2 or more. Therefore a 500kV line could be upwards of a million dollars a mile. Considering your transmission line may have to travel 50 or more miles to get to a main connection, you are looking at ten of millions of dollars, and that’s just for the transmission line. You will also need a collection station for the turbines, thousands of miles of underground cables, a power substation, reactive power support, communication lines, and a slew of other equipment and facilities. In short, its not easy or is it cheap. Oh, and don’t forget right of way (ROW) many project are delayed or cancelled because land owners down want a 500kV transmission line on their farm land.
This post was not meant to be a shooting down of all renewables goals. To continue to use power the way we do, we have no choice but to use renewables and keep finding ways to increase our utilization of them. What we need to keep in mind though is this is not simple, nor straight forward. Aside from the financial piece, there is significant research and engineering needed in the area to make this happen. So I urge you when you talk/think about renewables, be skeptical, ask good questions, and be patient. Hopefully now you have at least a slightly better understanding on where we are and how much it will take to get where we want to go. Now you know – and knowing is half the battle…